fix asymetric IPV4/IPV6 mtu discovery

when the remote (server) has a IPV6 interface selected by ICE,
and local (client) has a IPV4 selected, the path MTU discovery triggers
errors for packets too big on server side because of different
IP headers overhead.
Hence we have to signal to the TLS session to reduce the MTU discovery
packets payload size on client side accordingly.

[GR: use GenericSocket class to export local/remote address]

Change-Id: I724b97c9ccd5e7240a87fb60ef000b4485710131
Signed-off-by: Guillaume Roguez <guillaume.roguez@savoirfairelinux.com>
Reviewed-by: Guillaume Roguez <guillaume.roguez@savoirfairelinux.com>