* 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
* Adds Lua methods 'set_rotation()' and 'get_rotation'. Also changed some method names to be more clear. Instead of an f32 being sent over network for yaw, now a v3f is sent for rotation on xyz axes. Perserved Lua method set_yaw/setyaw so that old mods still work, other wise to set yaw they would need to switch to set_rotation(0, yaw, 0).
