BUG/MEDIUM: mux-quic: fix nb_hreq decrement

BUG/MEDIUM: mux-quic: fix nb_hreq decrement

nb_hreq is a counter on qcc for active HTTP requests. It is incremented
for each qcs where a full HTTP request was received. It is decremented
when the stream is closed locally :
- on HTTP response fully transmitted
- on stream reset

A bug will occur if a stream is resetted without having processed a full
HTTP request. nb_hreq will be decremented whereas it was not
incremented. This will lead to a crash when building with
DEBUG_STRICT=2. If BUG_ON_HOT are not active, nb_hreq counter will wrap
which may break the timeout logic for the connection.

This bug was triggered on haproxy.org. It can be reproduced by
simulating the reception of a STOP_SENDING frame instead of a STREAM one
by patching qc_handle_strm_frm() :

+       if (quic_stream_is_bidi(strm_frm->id))
+               qcc_recv_stop_sending(qc->qcc, strm_frm->id, 0);
+       //ret = qcc_recv(qc->qcc, strm_frm->id, strm_frm->len,
+       //               strm_frm->offset.key, strm_frm->fin,
+       //               (char *)strm_frm->data);

To fix this bug, a qcs is now flagged with a new QC_SF_HREQ_RECV. This
is set when the full HTTP request is received. When the stream is closed
locally, nb_hreq will be decremented only if this flag was set.

This must be backported up to 2.6.