Uses VMOD dynamic, and requires that the getdns library is installed
in the image running Varnish. This allows us to use dynamic.resolve(),
in particular so that TTLs from DNS are honored.

Currently sets ttl to a hard-wired value of 30s. Since the TTLs for
lookup are obtained from DNS, this actually sets the delay until
lookups are retried after negative results (default 1h).

The next step is to test and extend BackendConfig support to configure
properties of VMOD dynamic. That will make it possible to configure
the ttl value (although we might stay with a much shorter ttl than
1h).

Partially addresses gitlab issue #20.