MINOR: h2: make use of client-fin timeout after GOAWAY

MINOR: h2: make use of client-fin timeout after GOAWAY

At the moment, the "client" timeout is used on an HTTP/2 connection once
it's idle with no active stream. With this patch, this timeout is replaced
by client-fin once a GOAWAY frame is sent. This closely matches what is
done on HTTP/1 since the principle is the same, as it indicates a willing
ness to quickly close a connection on which we don't expect to see anything
anymore.