BUG/MEDIUM: mux-h2: put BE conn back in idle list after deferred shut

BUG/MEDIUM: mux-h2: put BE conn back in idle list after deferred shut

When a backend stream cannot send its RST_STREAM or ES at detach time
because the mux buffer is busy or full, or because the connection window
is exhausted, h2_detach() leaves early to keep the stream as an orphan.
This skips the idle/avail list management performed at the end of the
function. The orphan is later destroyed by h2_deferred_shut() once the
frame can be sent, but nothing makes the connection idle or available
again at this point.

If the orphan was the last stream of the connection, the connection is
then in no idle list, it is not released and it remains accounted in
srv->curr_used_conns. Its timeout is not armed either, because a backend
connection without stream only relies on idle-ping. So it cannot be
reused and it stays there until the server closes it or an error occurs,
holding an FD and preventing the server from being deleted.

To fix the issue, the idle/avail list management is extracted from
h2_detach() into a new h2_may_idle_conn() function, which is now also
called from h2_deferred_shut() after the orphan was destroyed and the
timeout was refreshed, as h2_detach() does.

In h2_deferred_shut(), the stream's session cannot be used, since it may
have been released in the mean time. The connection owner is used instead,
but only if the connection is still attached to its session, because
session_free() resets conn->owner only in this case. For a private
connection, if there is no session anymore when the last stream leaves,
the connection can no longer be reused and it is destroyed.

The orphans destroyed from h2s_wake_one_stream() are not covered here.
This mostly happens on connection errors or shutdowns, in which case the
connection is released anyway.

This bug exists since orphaned streams are destroyed from
h2_deferred_shut() with backend connections reuse. It could be backported
to all supported versions. Note that 5e7bb2063 ("BUG/MEDIUM: mux-h2:
refresh the connection timeout before parking it idle") reworked the
h2_detach() tail, so it should be backported first. However, it could be
good to wait for a period of observation first.