engine: add flb_engine_exit()
int flb_engine_start(struct flb_config *config);
int flb_engine_flush(struct flb_config *config,
struct flb_input_plugin *in_force);
+int flb_engine_exit(struct flb_config *config);
int flb_engine_shutdown(struct flb_config *config);
int flb_engine_destroy_tasks(struct mk_list *tasks);
