Add possibility to override worker thread allocation logic in WebSocketServer  #279

Hey, may I ask you for accepting a PR that does just 2 minor things, making

WebSocketServer#decoders 
WebSocketServer#queue

protected field/method in order to do what's described here in the server's description :
https://github.com/viagraphs/reactive-websocket
https://github.com/viagraphs/reactive-websocket/blob/master/jvm/src/main/scala/com/viagraphs/websocket/RxWebSocketServer.scala#L69

I'd like to contribute to Java-WebSocket in following several months when I'm gonna build a project around it.