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.cc

@@ -353,6 +353,7 @@ static MXS_ROUTER* createInstance(SERVICE* service, MXS_CONFIG_PARAMETER* params
     inst->maxwell_compat = params->get_bool("maxwell-compatibility");
     inst->trx_safe = params->get_bool("transaction_safety");
     inst->fileroot = params->get_c_str_copy("filestem");
+    inst->heartbeat_task_active = false;
 
     /* Server id */
     inst->serverid = params->get_integer("server_id");