* 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
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