Don't defer WebSocket opening

[slandelle]

We currently buffer WebSocket opening until first LastHttpContent reception with the UpgradeCallback .
This doesn't make sense, and forces us to buffer any frame that might be sent along with the upgrade response.

WebSocketHandler and WebSocketUpgradeHandler need serious clean up:

• Drop UpgradeHandler that's never used as an abstraction
• Perform upgrade/abort as soon as response is received
• Ignore LastHttpContent
• No need to buffer any frame