[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/jobs/base_workers_test.go

@@ -29,9 +29,9 @@ func TestSimpleWorkerPanic(t *testing.T) {
 		return true
 	}
 
-	mockStore.JobStore.On("UpdateStatusOptimistically", "job_id", model.JobStatusPending, model.JobStatusInProgress).Return(true, nil)
+	mockStore.JobStore.On("UpdateStatusOptimistically", "job_id", model.JobStatusPending, model.JobStatusInProgress).Return(&model.Job{Id: "job_id", Type: "job_type"}, nil)
 	mockStore.JobStore.On("UpdateOptimistically", mock.AnythingOfType("*model.Job"), model.JobStatusInProgress).Return(true, nil)
-	mockStore.JobStore.On("Get", mock.AnythingOfType("*request.Context"), "job_id").Return(nil, errors.New("test"))
+	mockStore.JobStore.On("UpdateStatus", "job_id", "success").Return(nil, errors.New("test"))
 	mockMetrics.On("IncrementJobActive", "job_type")
 	mockMetrics.On("DecrementJobActive", "job_type")
 	sWorker := NewSimpleWorker("test", jobServer, exec, isEnabled)