MINOR: quic-stream: refactor ack management

MINOR: quic-stream: refactor ack management

Acknowledge of STREAM has been complexified with the introduction of
stream multi buffers. Two functions are executing roughly the same set
of instructions in xprt_quic.c.

To simplify this, move the code complexity in a new function
qc_stream_desc_ack(). It will handle offset calculation, removal of
data, freeing oldest buffer and freeing stream instance if required.
The qc_stream_desc API is cleaner as qc_stream_desc_free_buf() ambiguous
function has been removed.