MINOR: mux-quic: implement MAX_STREAM_DATA emission

MINOR: mux-quic: implement MAX_STREAM_DATA emission

Send MAX_STREAM_DATA frames when at least half of the allocated
flow-control has been demuxed, frame and cleared. This is necessary to
support QUIC STREAM with received data greater than a buffer.

Transcoders must use the new function qcc_consume_qcs() to empty the QCS
buffer. This will allow to monitor current flow-control level and
generate a MAX_STREAM_DATA frame if required. This frame will be emitted
via qc_io_cb().