BUG/MEDIUM: mux_quic: adjust qcc_is_dead() to account detached streams

BUG/MEDIUM: mux_quic: adjust qcc_is_dead() to account detached streams

Muxes are responsible to release connections once they are inactive and
won't be reusable. In QUIC mux, such connections are detected via
qcc_is_dead(). The first precondition is that there is no more upper
streams attached. This was accounted via QCC <nb_sc> counter.

A special characteristic of QCS instances is that they can be in
detached state : upper stream has been removed but there is still data
to emit. Such QCS were not taken into account in qcc_is_dead(), so a
connection could be freed with some remaining data not yet emitted.

It is also not possible for QUIC MUX to simply look at the QCS tree to
determine if the connection is inactive. Indeed, some streams are opened
for protocol internal usage. This is the case for example with HTTP/3
unidirectional control stream or QPACK encoder/decoder streams. These
streams are never closed. In the end, only requests streams should be
taken into account for the connection activity.

This patch improves the situation by reworking <nb_hreq> QCC counter.
Previously, it served for http-request timeout implementation. However,
this timeout only relies on <opening_list> now. Thus, <nb_hreq> scope is
changed : it is now incremented via qcs_wait_http_req(), used by app
protocol layer once a request stream is identified. Decrement is
performed on qcs_free(), so this guarantees that a connection cannot be
freed anymore if request streams still exists, unless if inactivity
timeout fires. As such, <nb_hreq> now supersedes <nb_sc> entirely, so
the qcc_is_dead() can now relies on the former.

Along with this change, qcc_timeout_task() must be updated. Call to
qcc_is_dead() was unnecessary prior to this patch as timeout handling
was only active when no upper streams were attached. When tested, both
<nb_sc> and QCC <task> were already null, so a connection was always
released on timeout, as expected. With qcc_is_dead() now checking
<nb_hreq> instead, this is not always the case anymore. In fact, this
check is unnecessary as inactivity timeout serves precisely to free a
stucked connection with remaining data to emit.

This patch also has some impact on http-keep-alive timeout. Previously,
this timeout could be armed if only detached streams remained. Now, it
is only applicable if all QCS request instances are closed and freed.
Thus, qcc_reset_idle_start() is now closed directly on qcs_free().

Ideally this should be backported up to 2.6, or at least 2.8 as QUIC
experimental status was removed there.