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?

Start by reviewing h2's connection state machine alongside RFC 7540 section 6.8, focusing on the transition after sending or receiving GOAWAY. Reproduce the current prohibition on further traffic and determine the expected behavior; done means in-progress streams can complete while new streams remain prohibited, with tests covering both directions.