Added Handler.FastDirectSendingMode.

When FastDirectSendingMode is set to true, the send-threads are not started.
Messages are then sent directly from the multithreaded application to
the underlaying socket buffer. The Send method may block a short time
until the previous send operation has copied its data do the socket buffer.
There is a considerable speed increase for applications needing
fast roundtrip times and have a small number of clients:
    FastDirectSendingMode = false:   33 request+responses per second - caused by a delay of 10ms in every dequeue operation.
    FastDirectSendingMode = true:  2500 request+responses per second
Skip pending send operations when disconnecting a client.
Removed unused method 'DoReceive'.
Added static methods to shutdown Alchemy threads.
Documented the strange behavour of WebSocketServer.Dispose().
Fixed client disconnect sequence.