MINOR: quic: use separate global quic_conns FE/BE lists

MINOR: quic: use separate global quic_conns FE/BE lists

Each quic_conn instance is stored in a global list. Its purpose is to be
able to loop over all known connections during "show quic".

Split this into two separate lists for frontend and backend usage.
Another change is that closing backend connections do not move into
quic_conns_clo list. They remain instead in their original list. The
objective of this patch is to reduce the contention between the two
sides.

Note that this prevents backend connections to be listed in "show quic"
now. This will be adjusted in a future patch.