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

api4/system.go

@@ -492,12 +492,5 @@ func getServerBusyExpires(c *Context, w http.ResponseWriter, r *http.Request) {
 		c.SetPermissionError(model.PERMISSION_MANAGE_SYSTEM)
 		return
 	}
-
-	busy := c.App.Srv.Busy
-	sbs := &model.ServerBusyState{
-		Busy:       busy.IsBusy(),
-		Expires:    busy.Expires().Unix(),
-		Expires_ts: busy.Expires().UTC().Format("Mon Jan 2 15:04:05 -0700 MST 2006"),
-	}
-	w.Write([]byte(sbs.ToJson()))
+	w.Write([]byte(c.App.Srv.Busy.ToJson()))
 }