MINOR: mux-quic: extract code to build STREAM frames list

MINOR: mux-quic: extract code to build STREAM frames list

Extracts code responsible to generate STREAM, RESET_STREAM and
STOP_SENDING frames for each qcs instances registered in qcc send_list.
It is moved from qcc_io_send() to its owned new function
qcc_build_frms().

This commit does not bring functional change. It is a preparatory step
to adapt QUIC MUX send mechanism to allow reusing of qcc frms list
accross qcc_io_send() invokation.

As a side change, qcc_tx_frms_free() is renamed to qcc_clear_frms().
This better highlights its relationship with qcc_build_frms().

This should be bkacported up to 3.1.