Only add tasks once in binlogrouter

Adding the same task twice isn't allowed. The API of the housekeeper tasks
might have to be changed in a way that makes it possible for the caller to
know whether a task has been added.

server/modules/routing/binlogrouter/blr.hh

@@ -799,6 +799,7 @@ struct ROUTER_INSTANCE : public MXS_ROUTER
     unsigned int            long_burst; /*< Long burst for slave catchup */
     unsigned long           burst_size; /*< Maximum size of burst to send */
     unsigned long           heartbeat;  /*< Configured heartbeat value */
+    bool                    heartbeat_task_active;
     ROUTER_STATS            stats;      /*< Statistics for this router */
     int                     active_logs;
     int                     reconnect_pending;