BUG/MEDIUM: mux-h2: only close connection on request frames on closed streams

BUG/MEDIUM: mux-h2: only close connection on request frames on closed streams

A subtle bug was introduced with H2 on the backend. RFC7540 states that
an attempt to create a stream on an ID not higher than the max known is
a connection error. This was translated into rejecting HEADERS frames
for closed streams. But with H2 on the backend, if the client aborts
and causes an RST_STREAM to be emitted, the stream is effectively closed,
and if/once the server responds, it starts by emitting a HEADERS frame
with this ID thus it is interpreted as a connection error.

This test must of course consider the side the mux is installed on and
not take this for a connection error on responses.

The effect is that an aborted stream on an outgoing H2 connection, for
example due to a client stopping a transfer with option abortonclose
set, would lead to an abort of all other streams. In the logs, this
appears as one or several CD-- line(s) followed by one or several SD--
lines which are victims.

Thanks to Luke Seelenbinder for reporting this problem and providing
enough elements to help understanding how to reproduce it.

This fix must be backported to 1.9.