MINOR: mux-quic: do not send STREAM frames if already subscribe

MINOR: mux-quic: do not send STREAM frames if already subscribe

Do not built STREAM frames if MUX is already subscribed for sending on
lower layer. Indeed, this means that either socket currently encountered
a transient error or congestion window is full.

This change is an optimization which prevents to allocate and release a
series of STREAM frames for nothing under congestion.

Note that nothing is done for other frames (flow-control, RESET_STREAM
and STOP_SENDING). Indeed, these frames are not restricted by flow
control. However, this means that they will be allocated for nothing if
send is blocked on a transient error.

This should be backported up to 2.7.