Optimise creation of DM (#16819)

* Optimise creation of dm

* Handle direct channels with the same user

* Cover GetMany with specs and add it on tha cache layer as well

* Fix specs by handling user dming themselves

* Apply PR suggestions

* Apply PR suggestions

* Use require.NoError instead of require.Nil on userstore test

* Improve readability of GetOrCreateDirectChannel

* Apply PR suggestions

* Update layers

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

store/retrylayer/retrylayer.go

@@ -9428,6 +9428,26 @@ func (s *RetryLayerUserStore) GetKnownUsers(userID string) ([]string, error) {
 
 }
 
+func (s *RetryLayerUserStore) GetMany(ids []string) ([]*model.User, error) {
+
+	tries := 0
+	for {
+		result, err := s.UserStore.GetMany(ids)
+		if err == nil {
+			return result, nil
+		}
+		if !isRepeatableError(err) {
+			return result, err
+		}
+		tries++
+		if tries >= 3 {
+			err = errors.Wrap(err, "giving up after 3 consecutive repeatable transaction failures")
+			return result, err
+		}
+	}
+
+}
+
 func (s *RetryLayerUserStore) GetNewUsersForTeam(teamId string, offset int, limit int, viewRestrictions *model.ViewUsersRestrictions) ([]*model.User, error) {
 
 	tries := 0