onPing/onClose should be more robust and friendly while fully-compliant with the protocol

(see the originating discussion here:  http://mail.openjdk.java.net/pipermail/net-dev/2016-June/009875.html)

1. Change `sendClose(CloseCode, CharSequence)` to `sendClose(int, CharSequence)`
2. Remove `CloseCode`
3. Make both `sendClose` methods idempotent
4. Change `default void onClose(WebSocket webSocket, Optional<CloseCode> code, String reason) { }` to
`default CompletionStage<?> onClose(WebSocket webSocket, int statusCode, String reason) { return null; }`
5. Change `default CompletionStage<?> onPing(WebSocket webSocket, ByteBuffer message) {
            return webSocket.sendPong(message).thenRun(() -> webSocket.request(1));
        }
` to `default CompletionStage<?> onPing(WebSocket webSocket, ByteBuffer message) {
            return null;
        }`
6. Specify one of the (onClose|onError) is always invoked. e.g. onClose(1006)