HTTP/2 server-initiated streams

It would be nice if the HTTP/2 implementation supported server-initiated streams to a client, after the connection prefaces have been exchanged.

A quick look at what this would require is:
- to emit a "ready" event after the preface has been successfully received (both on client and on server). This would allow a server to know the first available moment to initiate a stream towards the client.
- to handle server-initiated streams in `HTTP2ClientSession.onHeaders()`, as currently they are dropped. The implementation would be trivial and basically be a copy of what the server does.