Rewrite WebSocketWriter to use StreamWriter

[asvetlov]

Now websocket writer has custom flow control.
It's maybe not bad but overcomplicates things.
Since #2836 is done websocket writer uses coroutines everywhere, the transition should be smooth.

No Public API changes but limit=DEFAUTLT_LIMIT and MSG_SIZE will be dropped.
I see related micro-optimizations made by @fafhrd91 and @pfreixes but I believe the performance impact will be negligible.

Talking about performance using bytearray as websocket message structure and adding Cython optimization for parsing/building websocket header can get better speed boost.