Implement websocket boostrapping for http2

[ttaym]

Contributes to #755

• HTTP/2 -> HTTP/1.1
• HTTP/2 stream FSM amendments
• Sec-WebSocket-Key insert
• HTTP/1.1 -> HTTP/2
• Sec-WebSocket-Accept strip
• Pairing server connection to client stream
• Websocket protocol amendments
• Gracefull shutting down stream

This PR adds CONNECT method and :protocol header field for HTTP/2 parsing.

CONNECT HTTP/2 messages with :protocol equals to websocket transformed to HTTP/1.1 messages for backend with method GET, headers Connection: upgrade and Upgrade: websocket.

Responses from backend with headers Connection: upgrade, Upgrade: websocket and status 101 transformed to :status 200 HTTP/2 responses to client.

When sending 200-success to CONNECT request with websocket :protocol we need not to send END_STREAM frame flag. So to gracefully shutting down stream we need to detect Close websocket control frame or rely on closing tcp connection from backend or closing stream from client.

Note that Sec-WebSocket-Accept stripped from HTTP/1.1 responses and Sec-WebSocket-Key added to HTTP/1.1 request to backend.

Signed-off-by: Aleksey Mikhaylov aym@tempesta-tech.com