MINOR: mux-quic: implement qstream send callback

MINOR: mux-quic: implement qstream send callback

Each time a QUIC frame is emitted, mux-quic layer is notified via a
callback to update the underlying QCS. For QUIC, this is performed via
qc_stream_desc element.

In QMux protocol, this can be simplified as there is no
qc_stream_desc/quic_conn layer interaction. Instead, each time snd_buf
is called, QCS can be updated immediately using its return value. This
is performed via a new function qstrm_ctrl_send().

Its work is similar to the QUIC equivalent but in a simpler mode. In
particular, sent data can be immediately removed from the Tx buffer as
there is no need for retransmission when running above TCP.