[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/app/export_test.go

@@ -941,27 +941,25 @@ func TestExportFileWarnings(t *testing.T) {
 
 			job, appErr := th.App.Srv().Jobs.CreateJob(th.Context, model.JobTypeExportProcess, nil)
 			require.Nil(t, appErr)
-			ok, appErr := th.App.Srv().Jobs.ClaimJob(job)
-			require.Nil(t, appErr)
-			require.True(t, ok)
-			job, appErr = th.App.Srv().Jobs.GetJob(th.Context, job.Id)
+			newJob, appErr := th.App.Srv().Jobs.ClaimJob(job)
 			require.Nil(t, appErr)
+			require.NotNil(t, newJob)
 
 			opts := model.BulkExportOpts{
 				IncludeAttachments: true,
 				CreateArchive:      true,
 			}
-			appErr = th.App.BulkExport(th.Context, exportFile, dir, job, opts)
+			appErr = th.App.BulkExport(th.Context, exportFile, dir, newJob, opts)
 			// should not get an error for the missing file
 			require.Nil(t, appErr)
 
 			// should get a warning instead:
 			testlib.AssertLog(t, buffer, mlog.LvlWarn.Name, "Unable to export file attachment")
 
-			// should get info in the job data:
-			job, appErr = th.App.Srv().Jobs.GetJob(th.Context, job.Id)
+			// should get info in the newJob data:
+			newJob, appErr = th.App.Srv().Jobs.GetJob(th.Context, newJob.Id)
 			require.Nil(t, appErr)
-			warnings, ok := job.Data["num_warnings"]
+			warnings, ok := newJob.Data["num_warnings"]
 			require.True(t, ok)
 			require.Equal(t, "1", warnings)