MINOR: quic: detect connection migration

MINOR: quic: detect connection migration

Detect connection migration attempted by the client. This is done by
comparing addresses stored in quic-conn with src/dest addresses of the
UDP datagram.

A new function qc_handle_conn_migration() has been added. For the
moment, no operation is conducted and the function will be completed
during connection migration implementation. The only notable things is
the increment of a new counter "quic_conn_migration_done".

This should be backported up to 2.7.