Interface support for trade service (Implemented for bitmex and okcoin/okex)

[declan94]

Add interface support for streaming trade service to get trading order information through WebSocket API.
Implemented the interface for bitmex and okcoin(okex).

[henryxwong]

Anything I can help with this PR? Code review or resolving the conflict?

[makarid]

I am waiting for this PR to merge in order to update the StreamingTradeService with getUserTrades implementation.

[mdvx]

How did you avoid this? WebSocketClientHandler.channelRead0

The GZIP code is for another Exchange, Kraken I think, but Upbit was sending as Binary frames too

    } else if (frame instanceof BinaryWebSocketFrame) {
        BinaryWebSocketFrame binaryFrame = (BinaryWebSocketFrame) frame;
        ByteBuf bf = binaryFrame.content();
        byte[] bytes = new byte[bf.capacity()];
        bf.readBytes(bytes);
        if (bytes[0] == (GZIPInputStream.GZIP_MAGIC & 0x00ff) && bytes[1] == (GZIPInputStream.GZIP_MAGIC & 0xff00) >> 8) {  // upbit GZIPInputStream.
            GZIPInputStream gzipInputStream = new GZIPInputStream(new ByteArrayInputStream(bytes));
            ByteArrayOutputStream result = new ByteArrayOutputStream();
            byte[] buffer = new byte[1024];
            int length;
            while ((length = gzipInputStream.read(buffer)) != -1) {
                result.write(buffer, 0, length);
            }
            handler.onMessage(result.toString());
        } else {
            handler.onMessage(new String(bytes));  // upbit
        }
    }

[badgerwithagun]

@declan94 , @makarid , @mdvx: I'm trying to get as many PRs as I can either merged or closed, so we can start working towards merging this library with XChange. What is the status of this? Is anyone still waiting for it? Are we close to being able to merge?

[badgerwithagun]

OK guys, I'm going to close this but add a reference to the README to say this PR can be used as a start point if anyone wants to add Bitmex/OKEX private channels in the future.