[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/store.go

@@ -783,7 +783,7 @@ type JobStore interface {
 	SaveOnce(job *model.Job) (*model.Job, error)
 	UpdateOptimistically(job *model.Job, currentStatus string) (bool, error)
 	UpdateStatus(id string, status string) (*model.Job, error)
-	UpdateStatusOptimistically(id string, currentStatus string, newStatus string) (bool, error)
+	UpdateStatusOptimistically(id string, currentStatus string, newStatus string) (*model.Job, error)
 	Get(c request.CTX, id string) (*model.Job, error)
 	GetAllByType(c request.CTX, jobType string) ([]*model.Job, error)
 	GetAllByTypeAndStatus(c request.CTX, jobType string, status string) ([]*model.Job, error)