MEDIUM: mux-quic: improve bidir STREAM frames sending

MEDIUM: mux-quic: improve bidir STREAM frames sending

The current implementation of STREAM frames emission has some
limitation. Most notably when we cannot sent all frames in a single
qc_send run.

In this case, frames are left in front of the MUX list. It will be
re-send individually before other frames, possibly another frame from
the same STREAM with new data. An opportunity to merge the frames is
lost here.

This method is now improved. If a frame cannot be send entirely, it is
discarded. On the next qc_send run, we retry to send to this position. A
new field qcs.sent_offset is used to remember this. A new frame list is
used for each qc_send.

The impact of this change is not precisely known. The most notable point
is that it is a more logical method of emission. It might also improve
performance as we do not keep old STREAM frames which might delay other
streams.