Avoid delays when inserting HTTP/2 handlers.

Motivation

Right now, we insert HTTP/2 handlers in a callback on a future that
is done very late. The result of this is that an entire ALPN negotiaton
_can_ complete before this callback is attached. That can in rare
cases cause the HTTP/2 handler to miss the server preamble, because
it gets added too late.

Modifications

This patch refactors the existing code to close that window. It does
so by passing a promise into the connection path and completing
that promise _on_ the event loop where we add the ALPN handlers, which
should ensure this will execute immediately when the ALPN negotiation
completes. Immportantly, we attach our promise callbacks to that
promise _before_ we hand it off, making sure the timing windows go
away.

Results

Timing window is closed

Author: Lukasa