BUG/MINOR: quic: fix race-condition on trace for CID retrieval

BUG/MINOR: quic: fix race-condition on trace for CID retrieval

quic_rx_pkt_retrieve_conn() is used when parsing a received datagram
from the listener socket. It returned the quic_conn instance
corresponding to the first packet DCID, unless it is mapped to another
thread.

As expected, global CID tree access is protected by a lock in the
function. However, there is a race condition due to the final trace
where qc instance is dereferenced outside of the lock. Fix this by
adding a new trace under lock protection and remove qc deferencement at
function end.

This may fix first crash of github issue #2607.

This must be backported up to 2.8.