OPTIM: quic: rework the QUIC RX code

OPTIM: quic: rework the QUIC RX code

Use an MPSC ring buffer to hold data for each datagram handler. Holding
this data in a per-handler buffer avoids the HoL blocking we experienced
when we had per-listener buffers with data from all threads mixed up
in them.

This also gets rid of the mt_list contention we were suffering before,
that was causing some threads to be stuck for a significant amount of
time, causing warnings and even crashes in some cases.