Otherwise there is a timing bug where if the client happens to respond quickly and the handshake thread happens to be
suspended between sending the message and setting the state.

This removes the need for `HandshakeObserver` as well as most methods on `Handshake`, since the information can now be
obtained through the future results.

Handshakes also register and unregister themselves as message handlers. This removes the need for `releaseHandshake()`,
as well as for `MapToolServerConnection` to keep track of handshakes.

The client can now be in one of these states: `New`; `Started`; `Connected`; `Closed`. When transitioning states, we
check that we are in an acceptable state or otherwise disallow the transition. This ensures that calls like `start()` or
`stop()` can only be effected once.

ServerCommandClientImpl will now avoid sending messages unless the client state is `Connected`. Among other things, this
prevents unwanted heartbeats from being sent during handshakes.

Instead, the sole observable player database (`PasswordFilePlayerDatabase`) will fire these
events. `PlayerDatabaseDialogController` is the only listener and now registers directly on the database.

The public API of `Players` has also been reduced, either by privatizing methods or removing trivial wrappers of its
player database.