Add prepareMessage and sendPrepared functions

[jgupta]

We could improve performance in pub-sub or broadcast cases if functions such as prepareMessage and sendPrepared are added (especially in case of compression)

https://github.com/uWebSockets/uWebSockets/issues/141
http://autobahn.ws/python/_modules/autobahn/websocket/interfaces.html#IWebSocketChannel.sendPreparedMessage

Describe what the feature does or link to an explanation of the feature.

[jgupta]

For e.g. a message X has to be sent to N number fo clients. By default, it would do same work (Framing, compression etc) N times.

prepareMessage: Prepare message to be sent, this would do framing, compression etc.
sendPrepared: Send already prepared message by skipping framing, compression assuming all of this is already done.

http://autobahn.ws/python/reference/autobahn.websocket.html#autobahn.websocket.interfaces.IWebSocketChannel.sendPreparedMessage
http://autobahn.ws/python/reference/autobahn.websocket.html#autobahn.websocket.IWebSocketChannel.sendPreparedMessage

https://github.com/uWebSockets/uWebSockets/issues/136
https://github.com/uWebSockets/uWebSockets/issues/141

[jgupta]

I understand. Is there any way we could improve performance when we are sending high throughput identical messages to over multiple clients?

Fixed by 804cb60.