Fix some shutdown issues in connection tracker code.

src/ssb.c

@@ -2565,6 +2565,11 @@ void tf_ssb_destroy(tf_ssb_t* ssb)
 	}
 }
 
+bool tf_ssb_is_shutting_down(tf_ssb_t* ssb)
+{
+	return ssb->shutting_down;
+}
+
 void tf_ssb_run(tf_ssb_t* ssb)
 {
 	uv_run(ssb->loop, UV_RUN_DEFAULT);