BUG/MINOR: fix memleak on quic-conn streams cleaning

BUG/MINOR: fix memleak on quic-conn streams cleaning

When freeing a quic-conn, the streams resources attached to it must be
cleared. This code is already implemented but the streams buffer was not
deallocated.

Fix this by using the function qc_stream_desc_free. This existing
function centralize all operations to properly free all streams
elements, attached both to the MUX and the quic-conn.

This fixes a memory leak which can happen for each released connection.