BUG/MINOR: quic: flag conn with CO_FL_FDLESS on backend side

BUG/MINOR: quic: flag conn with CO_FL_FDLESS on backend side

Connection struct defines an handle which can point to either a FD or a
quic_conn. On the latter case, CO_FL_FDLESS must be set. This is already
the case on frontend side.

This patch fixes QUIC backend support. Before setting connection handle
member to a quic_conn instance, ensure that CO_FL_FDLESS flag is set on
the connection.

Prior to this patch, crash can occur in "show sess all".

No need to backport.