1. Implement thread-safe ringbuffer for websocket messages
This implements a ringbuffer that is used to decouple the raw websocket
connection from the messages that the game handler handles.
2. Change websocket handling
With this commit, we stop waiting for the websocket connection to be
established before the game starts.
Now, the Connection type is responsible for waiting for the websocket
connection before writing.
Some bugs are still happening:
1. The rejoining client is not told the state of the board
2. Invalid moves are not handled by the client (not sure why though)
3. The still-connected client should be told, that the opponent
disconnected. Then the client should show the passphrase again
3. Introduce method to send status of board and player
4. Reconnect works (kind of)
With the right changes in the client, the reconnect works (but only for
the first time).
WARNING:
At the moment, we will create a new player whenever connection wants to
join a private game. This will also clear all the disconnect callbacks
that we set in the player.
In order to simplify special moves like en passant or castling for the
client, we want to deliver the board state after every move (and not
only start square and end square).
With PGN we can encode a chess position into a string.
This commit implies changes to logic of the pieces' shortnames. This
will break the client/server connection (at least for promotions).
