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