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

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