BUG/MINOR: mux-h2: make sure request trailers on aborted streams don't break the...

BUG/MINOR: mux-h2: make sure request trailers on aborted streams don't break the connection

We used to respond a connection error in case we received a trailers
frame on a closed stream, but it's a problem to do this if the error
was caused by a reset because the sender has not yet received it and
is just a victim of the timing. Thus we must not close the connection
in this case.

This patch may be backported to 1.9 but then it requires the following
previous ones :
   MINOR: h2: add a generic frame checker
   MEDIUM: mux-h2: check the frame validity before considering the stream state
   CLEANUP: mux-h2: remove stream ID and frame length checks from the frame parsers