Fix various player save issues (performance penalty on sql backends + bugs)

* PostgreSQL & SQLite3 doesn't setModified(false) on RemotePlayer, then player is saved on each server save call. This results in heavy useless writes.
* PostgreSQL & SQLite3 ack engine meta write whereas db commit hasn't been performed. If commit failed write has failed. We mustn't notify engine write is done.
* serializing player meta must not setModified(false) because it didn't ensure write has been done
* add RemotePlayer::on_successfull_save callback to do the flag update on a successful save

src/remoteplayer.h

@@ -139,6 +139,8 @@ public:
 
 	void setPeerId(session_t peer_id) { m_peer_id = peer_id; }
 
+	void on_successful_save();
+
 private:
 	/*
 		serialize() writes a bunch of text that can contain