Initially to publish workqueue metrics.

Workaround for Varnish issue #2880:
https://github.com/varnishcache/varnish-cache/issues/2880

Refactor the test sources a bit while we're here.

Also shorten some names that predictably include '.' -- Service
backends and Host strings.

'compare' can be one of equal, not-equal, match or non-match.

'value' is either a fixed string for (in)equality tests, or a
regex to test for (non-)match.

This makes it possible to set values for the invocation via env
variables in the pod template, in particular to use PROXY.

This has required the introduction of pkg/interfaces, defining an
interface for Service Event generation, so that the Varnish controller
can hold a reference to IngressController, without introducing
import cycles for pkg/varnish and pkg/controller.

This has brought in a refactoring of controller/worker, introducing
the SyncObj to pass into the queue to workers. Now workers don't
have to guess at whether they're adding, updating or deleting.

And remove some dead code while we're at it.

Some elements were changed to be unexported, since they didn't need
to be exported or publicly documented.