Fix `autosave_interval` not being triggered by messages being delivered.

Closes #1726. Thanks to nduhme.

ChangeLog.txt

@@ -9,6 +9,8 @@ Broker:
 - Fix bridge topic remapping when using "" as the topic. Closes #1749.
 - Fix messages being queued for disconnected bridges when clean start was
   set to true. Closes #1729.
+- Fix `autosave_interval` not being triggered by messages being delivered.
+  Closes #1726.
 
 Client library:
 - Improved documentation around connect callback return codes. Close #1730.

src/database.c

@@ -339,6 +339,9 @@ int db__message_delete_outgoing(struct mosquitto_db *db, struct mosquitto *conte
 		}
 		db__message_dequeue_first(context, &context->msgs_out);
 	}
+#ifdef WITH_PERSISTENCE
+	db->persistence_changes++;
+#endif
 
 	return MOSQ_ERR_SUCCESS;
 }