REORG/MEDIUM: connection: introduce the notion of connection handle

REORG/MEDIUM: connection: introduce the notion of connection handle

Till now connections used to rely exclusively on file descriptors. It
was planned in the past that alternative solutions would be implemented,
leading to member "union t" presenting sock.fd only for now.

With QUIC, the connection will need to continue to exist but will not
rely on a file descriptor but a connection ID.

So this patch introduces a "connection handle" which is either a file
descriptor or a connection ID, to replace the existing "union t". We've
now removed the intermediate "struct sock" which was never used. There
is no functional change at all, though the struct connection was inflated
by 32 bits on 64-bit platforms due to alignment.