Simplify failed master detection

No longer depends on monitor events as the other operations do not
either. The active_event/new_event detection was removed, as these
only protect against a rare situation. A similar feature which
protects all the cluster modifications will be implemented later.

server/core/server.cc

@@ -162,7 +162,6 @@ SERVER* server_alloc(const char *name, MXS_CONFIG_PARAMETER* params)
     server->persistmax = 0;
     server->last_event = SERVER_UP_EVENT;
     server->triggered_at = 0;
-    server->active_event = false;
     server->status = SERVER_RUNNING;
     server->maint_request = MAINTENANCE_NO_CHANGE;
     server->version_string[0] = '\0';