MEDIUM: h2: partial implementation of h2_detach()

MEDIUM: h2: partial implementation of h2_detach()

This does the very minimum required to release a stream and/or a connection
upon the stream's request. The only thing is that it doesn't kill the
connection unless it's already closed or in error or the stream ID reached
the one specified in GOAWAY frame. We're supposed to arm a timer to close
after some idle timeout but it's not done.