Implement HTTP/2 CONNECT

[essen]

The branch http2-connect has been merged. Some points remain for improved support though.

• HTTP/2 timeouts are not propagated to the right layer. This will require Cowlib changes, hopefully not breaking changes.
• Events are for the most part not triggered.
• The set_cookie command is ignored when a tunnel uses it, and tunnels do not store cookie as a result. The set_cookie commands must be passed all the way down to the gun module.
• The internals could use some documentation to remember how it all works, particularly around the TLS handling, with a distinction between HTTP/1.1 / SOCKS tunnels vs tunnels that involve HTTP/2.
• Sometimes gun:stream_info will not return the reply_to pid for tunneled streams, it will return undefined. The switch_protocol command should be changed to add the reply_to value.
• There's no error handling therefore errors will probably lead to crashes. The same applies to the connection being closed by the remote endpoint.
• It is unclear how keepalive should be handled for tunnels.
• There's no support for graceful shutdown for tunnel streams.
• Graceful shutdown of the entire connection will not gracefully shut down tunnels.
• Flow control has not been implemented. Thought need to be given over how it should be implemented, both for normal CONNECT and for HTTP/2 Websocket.
• There seems to be no support for automatic transport detection based on the port number (like gun:open).

The code could also really use a good cleanup and todos to be solved.

[essen]

HTTP/2 timeouts are now properly propagated.

[essen]

Events are now for the most part triggered.

[essen]

I've reworked cookie handling for tunnels. The set_cookie command was removed and instead the cookie store is passed around where necessary. We no longer need a commands queue and whatnot.

[essen]

I've added flow control as a point to be tested and fixed. It also concerns Websocket over HTTP/2.

[essen]

https://github.com/ninenines/gun/blob/master/doc/src/guide/internals_tls_over_tls.asciidoc Feedback welcome.

[essen]

Added "There seems to be no support for automatic transport detection based on the port number (like gun:open)."