Optimize TeamsUnread (#19507)

The GetTeamsUnreadForUser call would be called for every team switch.
In CRT mode, it would make a separate store call for every team, which
would run the 3 aggregate SQL queries in GetThreadsForUser.

This is suboptimal because the complexity is linearly proportional
to the number of teams.

We make the following optimizations:
1. Change the query to a single one which aggregates all teams.
2. The query originally used just 2 out of the 3 queries, so one
query was fully redundant. We remove that query in the new one.
3. Further analysis was done whether it makes sense to run the 2
queries synchronously or not. The load-tests didn't show any degradation
in running them concurrently, so we keep the same behavior.

```release-note
NONE
```

store/timerlayer/timerlayer.go

@@ -8295,6 +8295,22 @@ func (s *TimerLayerThreadStore) GetPosts(threadID string, since int64) ([]*model
 	return result, err
 }
 
+func (s *TimerLayerThreadStore) GetTeamsUnreadForUser(userID string, teamIDs []string) (map[string]*model.TeamUnread, error) {
+	start := timemodule.Now()
+
+	result, err := s.ThreadStore.GetTeamsUnreadForUser(userID, teamIDs)
+
+	elapsed := float64(timemodule.Since(start)) / float64(timemodule.Second)
+	if s.Root.Metrics != nil {
+		success := "false"
+		if err == nil {
+			success = "true"
+		}
+		s.Root.Metrics.ObserveStoreMethodDuration("ThreadStore.GetTeamsUnreadForUser", success, elapsed)
+	}
+	return result, err
+}
+
 func (s *TimerLayerThreadStore) GetThreadFollowers(threadID string, fetchOnlyActive bool) ([]string, error) {
 	start := timemodule.Now()