Support graceful connection close

RFC 7540 section 6.8 seems to indicate that traffic can continue flowing in both directions after a GOAWAY frame, with the only restriction being that the recipient can't open new streams after receiving the GOAWAY:

   A GOAWAY frame might not immediately precede closing of the
   connection; a receiver of a GOAWAY that has no more use for the
   connection SHOULD still send a GOAWAY frame before terminating the
   connection.
...
   Activity on streams numbered lower or equal to the last stream
   identifier might still complete successfully.  The sender of a GOAWAY
   frame might gracefully shut down a connection by sending a GOAWAY
   frame, maintaining the connection in an "open" state until all in-
   progress streams complete.

But, h2's connection state machine immediately transitions to CLOSED upon sending or receiving a GOAWAY, effectively prohibiting any further traffic. Would you be interested in a patch to fix this, or is there some disconnect I'm missing between the wording of the RFC and actual implementations?

Inizia esaminando la macchina a stati della connessione di h2 insieme alla sezione 6.8 di RFC 7540, concentrandoti sulla transizione dopo l’invio o la ricezione di GOAWAY. Riproduci il divieto attuale di ulteriore traffico e determina il comportamento previsto; il lavoro sarà considerato completato quando gli stream in corso potranno terminare mentre i nuovi stream resteranno vietati, con test che coprano entrambe le direzioni.