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
```

app/team.go

@@ -1565,11 +1565,13 @@ func (a *App) GetTeamsUnreadForUser(excludeTeamId string, userID string, include
 		return tu
 	}
 
+	teamIDs := make([]string, 0, len(data))
 	for i := range data {
 		id := data[i].TeamId
 		if mu, ok := membersMap[id]; ok {
 			membersMap[id] = unreads(data[i], mu)
 		} else {
+			teamIDs = append(teamIDs, id)
 			membersMap[id] = unreads(data[i], &model.TeamUnread{
 				MsgCount:           0,
 				MentionCount:       0,
@@ -1584,18 +1586,22 @@ func (a *App) GetTeamsUnreadForUser(excludeTeamId string, userID string, include
 
 	includeCollapsedThreads = includeCollapsedThreads && *a.Config().ServiceSettings.CollapsedThreads != model.CollapsedThreadsDisabled
 
-	for _, member := range membersMap {
-		if includeCollapsedThreads {
-			data, err := a.Srv().Store.Thread().GetThreadsForUser(userID, member.TeamId, model.GetUserThreadsOpts{TotalsOnly: true, TeamOnly: true})
-			if err != nil {
-				return nil, model.NewAppError("GetTeamsUnreadForUser", "app.team.get_unread.app_error", nil, err.Error(), http.StatusInternalServerError)
-			}
-			member.ThreadCount = data.TotalUnreadThreads
-			member.ThreadMentionCount = data.TotalUnreadMentions
+	if includeCollapsedThreads {
+		teamUnreads, err := a.Srv().Store.Thread().GetTeamsUnreadForUser(userID, teamIDs)
+		if err != nil {
+			return nil, model.NewAppError("GetTeamsUnreadForUser", "app.team.get_unread.app_error", nil, err.Error(), http.StatusInternalServerError)
+		}
+		for teamID, member := range membersMap {
+			if _, ok := teamUnreads[teamID]; ok {
+				member.ThreadCount = teamUnreads[teamID].ThreadCount
+				member.ThreadMentionCount = teamUnreads[teamID].ThreadMentionCount
+			}
 		}
-		members = append(members, member)
 	}
 
+	for _, member := range membersMap {
+		members = append(members, member)
+	}
 	return members, nil
 }