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/database/database-files.cpp

@@ -105,7 +105,8 @@ void PlayerDatabaseFiles::savePlayer(RemotePlayer *player)
 	if (!fs::safeWriteToFile(path, ss.str())) {
 		infostream << "Failed to write " << path << std::endl;
 	}
-	player->setModified(false);
+
+	player->on_successful_save();
 }
 
 bool PlayerDatabaseFiles::removePlayer(const std::string &name)

src/database/database-postgresql.cpp

@@ -527,8 +527,9 @@ void PlayerDatabasePostgreSQL::savePlayer(RemotePlayer *player)
 		};
 		execPrepared("save_player_metadata", 3, meta_values);
 	}
-	sao->getMeta().setModified(false);
 	endSave();
+
+	player->on_successful_save();
 }
 
 bool PlayerDatabasePostgreSQL::loadPlayer(RemotePlayer *player, PlayerSAO *sao)

src/database/database-sqlite3.cpp

@@ -528,9 +528,10 @@ void PlayerDatabaseSQLite3::savePlayer(RemotePlayer *player)
 		sqlite3_vrfy(sqlite3_step(m_stmt_player_metadata_add), SQLITE_DONE);
 		sqlite3_reset(m_stmt_player_metadata_add);
 	}
-	sao->getMeta().setModified(false);
 
 	endSave();
+
+	player->on_successful_save();
 }
 
 bool PlayerDatabaseSQLite3::loadPlayer(RemotePlayer *player, PlayerSAO *sao)

src/remoteplayer.cpp

@@ -79,8 +79,6 @@ void RemotePlayer::serializeExtraAttributes(std::string &output)
 	}
 
 	output = fastWriteJson(json_root);
-
-	m_sao->getMeta().setModified(false);
 }
 
 
@@ -225,3 +223,10 @@ const RemotePlayerChatResult RemotePlayer::canSendChatMessage()
 	m_chat_message_allowance -= 1.0f;
 	return RPLAYER_CHATRESULT_OK;
 }
+
+void RemotePlayer::on_successful_save()
+{
+	setModified(false);
+	if (m_sao)
+		m_sao->getMeta().setModified(false);
+}

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