disconnected clients: Add reconnect task event (#12133)

* Add TaskClientReconnectedEvent constant
* Add allocRunner.Reconnect function to manage task state manually
* Removes server-side push

command/alloc_status.go

@@ -538,6 +538,8 @@ func buildDisplayMessage(event *api.TaskEvent) string {
 		desc = event.DriverMessage
 	case api.TaskLeaderDead:
 		desc = "Leader Task in Group dead"
+	case api.TaskClientReconnected:
+		desc = "Client reconnected"
 	default:
 		desc = event.Message
 	}