MINOR: mux-quic: clean up qcs Rx buffer allocation API

MINOR: mux-quic: clean up qcs Rx buffer allocation API

Replaces qcs_get_buf() function which naming does not reflect its
purpose. Add a new function qcc_get_stream_rxbuf() which allocate if
needed <qcs.rx.app_buf> and returns the buffer pointer. This function is
reserved for application protocol layer. This buffer is then accessed by
stconn layer.

For other qcs_get_buf() invocation which was used in effect for a local
buffer, replace these by a plain b_alloc().