When a client connects, the server tries to resolve the IP address
of the client.  In the beginning, this was done in a blocking way.
Bug 627 added non-blocking lookup of IPv4 addresses, but IPv6
adresses are still looked up in a blocking way.

We need to add IPv6 support to adns and possibly the liboop adapter.
(If we are lucky, newer versions of adns have the necessary support.
Check that.)

There is code in src/libraries/libisc-new/src/isc_socket.c that
checks for this condition:

      /* adns does not yet support IPv6 addresses, so if this is an
     IPv6 socket we have to use the old blocking method of host
     name lookup. */