[MM-63314] Fix ClaimJob in HA environments (#30383)

* ClaimJob now returns newly claimed job

* internal code affected by change

* test changes required

* two branches: for mysql, use transaction; for postgres, use returning

* two branches: for mysql, use transaction; for postgres, use returning

* use same millis value for LastActivityAt and StartAt

* blank commit

---------

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

server/channels/store/retrylayer/retrylayer.go

@@ -6384,7 +6384,7 @@ func (s *RetryLayerJobStore) UpdateStatus(id string, status string) (*model.Job,
 
 }
 
-func (s *RetryLayerJobStore) UpdateStatusOptimistically(id string, currentStatus string, newStatus string) (bool, error) {
+func (s *RetryLayerJobStore) UpdateStatusOptimistically(id string, currentStatus string, newStatus string) (*model.Job, error) {
 
 	tries := 0
 	for {