BUG/MINOR mux-quic: apply correctly timeout on output pending data

BUG/MINOR mux-quic: apply correctly timeout on output pending data

When no stream is attached, mux layer is responsible to maintain a
timeout. The first criteria is to apply client/server timeout if there
is still data waiting for emission.

Previously, <hreq> qcc member was used to determine this state. However,
this only covers bidirectional streams. Fix this by testing if
<send_list> is empty or not. This is enough to take into account both
bidi and uni streams.

Theorically, this should be backported to every stable versions.
However, send-list is not available on 2.6 and there is no alternative
to quickly determine if there is waiting output data. Thus, it's better
to backport it up to 2.8 only.