MM-40302: CRT, fix performance of MarkAllAsReadInChannels (#19566)

* deadcode: remove UpdateChannelLastViewedAt

* deadcode: remove ThreadStore.(Save(Multiple)|Update|Delete)

* deadcode: followThead in App.MarkChannelAsUnreadFromPost

* document ThreadMembership, Thread structs

* maintain LastUpdated consistently

Whenever we touch a `ThreadMembership` record, we should be setting `LastUpdated` to the current timestamp. The mobile client relies on this to detect changes to these records.

* simplify: never updateThreads from `App.MarkChannelAsUnreadFromPost`

Change all invocations of `ChannelStore.UpdateLastViewedAtPost` from `App.MarkChannelAsUnreadFromPost` to pass `updateThreads` as `false`. When `ChannelStore.UpdateLastViewedAtPost` was invoked with `updateThreads` as `true`, it would in turn call `ThreadStore.UpdateUnreadsByChannel` but pass `updateViewedTimestamp` as `false`. This effectively updated the `LastUpdated` field of the corresponding thread memberships but never touched any of the actual data (such as `LastViewed`).

The overall CRT feature continued to work, because `App.MarkChannelAsUnreadFromPost` directly updates the relevant thread memberships via `ThreadStore.MaintainMembership`.

* deadcode: updateThreads in ChannelStore.UpdateLastViewedAtPost

* simplify: never updateThreads from App.SendNotifications

Change all invocations of `ChannelStore.IncrementMentionCount` from
`App.SendNotifications` to pass `updateThreads` as `false`. When `ChannelStore.IncrementMentionCount` was invoked with `updateThreads` as `true`, it would in turn call `ThreadStore.UpdateUnreadsByChannel` but pass `updateViewedTimestamp` as `false`. This effectively updated the `LastUpdated` field of the corresponding thread memberships but never touched any of the actual data (such as `UnreadMentions`).

The overall CRT feature continued to work, because `App.SendNotifications` directly updates the relevant thread memberships mention counts via `ThreadStore.MaintainMembership`.

* deadcode: updateThreads in ChannelStore.IncrementMentionCount

* fix & rename ThreadStore.UpdateUnreadsByChannel

Rename `ThreadStore.UpdateUnreadsByChannel` to `ThreadStore.UpdateLastViewedByThreadIds`, making it unconditionally set the `LastViewed` for the given threads (as well as `LastUpdated`).

All previous invocations of this method that passed `updateViewedTimestamp` have been previously removed.

* unrelated gofmt -w -s changes to satisfy linter

* always set LastUpdated to model.GetMillis()

* deadcode: ThreadStore.SaveMembership

* fix TestMarkUnreadWithThreads

* MM-40302: CRT, use updateThreads param vs. MarkAllAsReadInChannels

`MarkAllAsReadInChannels` was the subject of a significant performance regression in v5.37 and is known to be very inefficient, by virtue of always writing to an ever increasing number of rows, and doing so on common events like simply viewing a channel.

Fortunately, `ChannelStore.UpdateLastViewedAt` already supported an `updateThreads` parameter that implemented the start of an improved algorithm: query the set of threads with newer posts, and then update only /those/. Missing was the need to reset the `UnreadMentions`, but thanks to the previous simplifications in #19523, we can make this change largely without impacting other semantics.

Fixes: https://mattermost.atlassian.net/browse/MM-40302

* fix MySQL

* remove another JOIN

* remove outdated comment

* unit tests

store/storetest/mocks/ThreadStore.go

@@ -15,29 +15,6 @@ type ThreadStore struct {
 	mock.Mock
 }
 
-// CollectThreadsWithNewerReplies provides a mock function with given fields: userId, channelIds, timestamp
-func (_m *ThreadStore) CollectThreadsWithNewerReplies(userId string, channelIds []string, timestamp int64) ([]string, error) {
-	ret := _m.Called(userId, channelIds, timestamp)
-
-	var r0 []string
-	if rf, ok := ret.Get(0).(func(string, []string, int64) []string); ok {
-		r0 = rf(userId, channelIds, timestamp)
-	} else {
-		if ret.Get(0) != nil {
-			r0 = ret.Get(0).([]string)
-		}
-	}
-
-	var r1 error
-	if rf, ok := ret.Get(1).(func(string, []string, int64) error); ok {
-		r1 = rf(userId, channelIds, timestamp)
-	} else {
-		r1 = ret.Error(1)
-	}
-
-	return r0, r1
-}
-
 // DeleteMembershipForUser provides a mock function with given fields: userId, postID
 func (_m *ThreadStore) DeleteMembershipForUser(userId string, postID string) error {
 	ret := _m.Called(userId, postID)
@@ -301,13 +278,13 @@ func (_m *ThreadStore) MaintainMembership(userID string, postID string, opts sto
 	return r0, r1
 }
 
-// MarkAllAsRead provides a mock function with given fields: userID, teamID
-func (_m *ThreadStore) MarkAllAsRead(userID string, teamID string) error {
-	ret := _m.Called(userID, teamID)
+// MarkAllAsRead provides a mock function with given fields: userID, threadIds
+func (_m *ThreadStore) MarkAllAsRead(userID string, threadIds []string) error {
+	ret := _m.Called(userID, threadIds)
 
 	var r0 error
-	if rf, ok := ret.Get(0).(func(string, string) error); ok {
-		r0 = rf(userID, teamID)
+	if rf, ok := ret.Get(0).(func(string, []string) error); ok {
+		r0 = rf(userID, threadIds)
 	} else {
 		r0 = ret.Error(0)
 	}
@@ -315,8 +292,8 @@ func (_m *ThreadStore) MarkAllAsRead(userID string, teamID string) error {
 	return r0
 }
 
-// MarkAllAsReadInChannels provides a mock function with given fields: userID, channelIDs
-func (_m *ThreadStore) MarkAllAsReadInChannels(userID string, channelIDs []string) error {
+// MarkAllAsReadByChannels provides a mock function with given fields: userID, channelIDs
+func (_m *ThreadStore) MarkAllAsReadByChannels(userID string, channelIDs []string) error {
 	ret := _m.Called(userID, channelIDs)
 
 	var r0 error
@@ -329,6 +306,20 @@ func (_m *ThreadStore) MarkAllAsReadInChannels(userID string, channelIDs []strin
 	return r0
 }
 
+// MarkAllAsReadByTeam provides a mock function with given fields: userID, teamID
+func (_m *ThreadStore) MarkAllAsReadByTeam(userID string, teamID string) error {
+	ret := _m.Called(userID, teamID)
+
+	var r0 error
+	if rf, ok := ret.Get(0).(func(string, string) error); ok {
+		r0 = rf(userID, teamID)
+	} else {
+		r0 = ret.Error(0)
+	}
+
+	return r0
+}
+
 // MarkAsRead provides a mock function with given fields: userID, threadID, timestamp
 func (_m *ThreadStore) MarkAsRead(userID string, threadID string, timestamp int64) error {
 	ret := _m.Called(userID, threadID, timestamp)
@@ -399,20 +390,6 @@ func (_m *ThreadStore) PermanentDeleteBatchThreadMembershipsForRetentionPolicies
 	return r0, r1, r2
 }
 
-// UpdateLastViewedByThreadIds provides a mock function with given fields: userId, threadIds, timestamp
-func (_m *ThreadStore) UpdateLastViewedByThreadIds(userId string, threadIds []string, timestamp int64) error {
-	ret := _m.Called(userId, threadIds, timestamp)
-
-	var r0 error
-	if rf, ok := ret.Get(0).(func(string, []string, int64) error); ok {
-		r0 = rf(userId, threadIds, timestamp)
-	} else {
-		r0 = ret.Error(0)
-	}
-
-	return r0
-}
-
 // UpdateMembership provides a mock function with given fields: membership
 func (_m *ThreadStore) UpdateMembership(membership *model.ThreadMembership) (*model.ThreadMembership, error) {
 	ret := _m.Called(membership)