MM-8607: Add ability to turn off non-critical services when under load; cluster support (#13267)

* MM-8607: Add cluster support for server busy status

MM-8607: Busy.Cluster should be non-exported

MM-8607: rename logging field seconds -> expires_sec

MM-8607: each node clears its own busy flag

MM-8607: ensure unit tests are not sensitive to test machine speed

* MM-8607: chg comment to force CI rebuild

app/admin.go

@@ -218,3 +218,13 @@ func (a *App) TestEmail(userId string, cfg *model.Config) *model.AppError {
 
 	return nil
 }
+
+// ServerBusyStateChanged is called when a CLUSTER_EVENT_BUSY_STATE_CHANGED is received.
+func (a *App) ServerBusyStateChanged(sbs *model.ServerBusyState) {
+	a.Srv.Busy.ClusterEventChanged(sbs)
+	if sbs.Busy {
+		mlog.Warn("server busy state activitated via cluster event - non-critical services disabled", mlog.Int64("expires_sec", sbs.Expires))
+	} else {
+		mlog.Info("server busy state cleared via cluster event - non-critical services enabled")
+	}
+}