MEDIUM: mux-h2: decode HEADERS frames before allocating the stream

MEDIUM: mux-h2: decode HEADERS frames before allocating the stream

It's hard to recover from a HEADERS frame decoding error after having
already created the stream, and it's not possible to recover from a
stream allocation error without dropping the connection since we can't
maintain the HPACK context, so let's decode it before allocating the
stream, into a temporary buffer that will then be offered to the newly
created stream.