MINOR: quic: ignore address migration during handshake

MINOR: quic: ignore address migration during handshake

QUIC protocol support address migration which allows to maintain the
connection even if client has changed its network address. This is done
through address migration.

RFC 9000 stipulates that address migration is forbidden before handshake
has been completed. Add a check for this : drop silently every datagram
if client network address has changed until handshake completion.

This commit is one of the first steps towards QUIC connection migration
support.

This should be backported up to 2.7.