Fix some shutdown issues in connection tracker code.

src/ssb.h

@@ -144,6 +144,13 @@ tf_ssb_t* tf_ssb_create(uv_loop_t* loop, JSContext* context, const char* db_path
 */
 void tf_ssb_destroy(tf_ssb_t* ssb);
 
+/**
+** Checking if the SSB instance is in the process of shutting down.
+** @param ssb The SSB instance.
+** @return true If the SSB instance is shutting down.
+*/
+bool tf_ssb_is_shutting_down(tf_ssb_t* ssb);
+
 /**
 ** Start optional periodic work.
 ** @param ssb The SSB instance.