MM-55295: Improve DB performance of some APIs (#25318)

Load tests show that channelstore.GetMember and
channelstore.GetMembersForUser are among the chief
queries that take up CPU in the DB.

In this PR, we attempt some strategic optimizations to
reduce/optimize calls to channelstore.GetMember

1. Optimize  `(a *App) HasPermissionToChannel`

We replace GetChannelMember with GetAllChannelMembersForUser
because it's cache backed. So although it gets more data,
it does not hit the DB and saves some latency.

2. Optimize getPostsForChannelAroundLastUnread

We repace getChannelMember with getChannelMemberOnly
which is a lite version of the store call which queries
just the ChannelMembers table. This is because
in the app layer, we just use the LastViewedAt attribute.
Therefore, there is no reason to join with 5 tables when
a single table can do the job.

3. Optimize publishWebsocketEventForPermalinkPost

We use GetAllChannelMembersById instead of GetChannelMembersPage
which again joins with a lot of other tables.

4. Optimize countMentionsFromPost

Again, we replace GetChannelMember which is a costly call joining
multiple tables, with GetAllChannelMembersNotifyPropsForChannel
which is cache-backed and gives us just what we need in the app
layer - notify props.

```release-note
Make small optimizations in several DB calls:
- App.HasPermissionToChannel
- getPostsForChannelAroundLastUnread
- publishWebsocketEventForPermalinkPost
- countMentionsFromPost
```

https://mattermost.atlassian.net/browse/MM-55295

---------

Co-authored-by: Mattermost Build <build@mattermost.com>

server/channels/store/timerlayer/timerlayer.go

@@ -907,10 +907,10 @@ func (s *TimerLayerChannelStore) GetAll(teamID string) ([]*model.Channel, error)
 	return result, err
 }
 
-func (s *TimerLayerChannelStore) GetAllChannelMembersById(id string) ([]string, error) {
+func (s *TimerLayerChannelStore) GetAllChannelMemberIdsByChannelId(id string) ([]string, error) {
 	start := time.Now()
 
-	result, err := s.ChannelStore.GetAllChannelMembersById(id)
+	result, err := s.ChannelStore.GetAllChannelMemberIdsByChannelId(id)
 
 	elapsed := float64(time.Since(start)) / float64(time.Second)
 	if s.Root.Metrics != nil {
@@ -918,7 +918,7 @@ func (s *TimerLayerChannelStore) GetAllChannelMembersById(id string) ([]string,
 		if err == nil {
 			success = "true"
 		}
-		s.Root.Metrics.ObserveStoreMethodDuration("ChannelStore.GetAllChannelMembersById", success, elapsed)
+		s.Root.Metrics.ObserveStoreMethodDuration("ChannelStore.GetAllChannelMemberIdsByChannelId", success, elapsed)
 	}
 	return result, err
 }
@@ -1451,6 +1451,22 @@ func (s *TimerLayerChannelStore) GetMemberForPost(postID string, userID string)
 	return result, err
 }
 
+func (s *TimerLayerChannelStore) GetMemberOnly(ctx context.Context, channelID string, userID string) (*model.ChannelMember, error) {
+	start := time.Now()
+
+	result, err := s.ChannelStore.GetMemberOnly(ctx, channelID, userID)
+
+	elapsed := float64(time.Since(start)) / float64(time.Second)
+	if s.Root.Metrics != nil {
+		success := "false"
+		if err == nil {
+			success = "true"
+		}
+		s.Root.Metrics.ObserveStoreMethodDuration("ChannelStore.GetMemberOnly", success, elapsed)
+	}
+	return result, err
+}
+
 func (s *TimerLayerChannelStore) GetMembers(channelID string, offset int, limit int) (model.ChannelMembers, error) {
 	start := time.Now()