BUG/MINOR: mux-quic: Fix memleak on QUIC stream buffer for unacknowledged data

BUG/MINOR: mux-quic: Fix memleak on QUIC stream buffer for unacknowledged data

Some clients send CONNECTION_CLOSE frame without acknowledging the STREAM
data haproxy has sent. In this case, when closing the connection if
there were remaining data in QUIC stream buffers, they were not released.

Add a <closing> boolean option to qc_stream_desc_free() to force the
stream buffer memory releasing upon closing connection.

Thank you to Tristan for having reported such a memory leak issue in GH #1801.

Must be backported to 2.6.