Previously we've been incorrectly transtitioning to CLOS_REM on
END_HEADERS, which prevents us from seeing if a client incorrectly
transmitted a DATA frame on a closed stream.

This slightly complicates things in that we can now be in state OPEN
with an inactive hpack decoding state, and we need to make sure on
cleanup if that has already been finalized.

This would be simpler if the h/2 spec had split the OPEN state in two
parts, with an extra state transition on END_HEADERS.

Again big thanks to @xcir for his help in diagnosing this.

Fixes: #2623