Fix binlogrouter slave heartbeat task addition

The task was added multiple times when only one task is needed.

server/modules/routing/binlogrouter/blr.hh

@@ -800,6 +800,7 @@ struct ROUTER_INSTANCE : public MXS_ROUTER
     unsigned long           burst_size; /*< Maximum size of burst to send */
     unsigned long           heartbeat;  /*< Configured heartbeat value */
     bool                    heartbeat_task_active;
+    bool                    slave_heartbeat_task_active;
     ROUTER_STATS            stats;      /*< Statistics for this router */
     int                     active_logs;
     int                     reconnect_pending;