Before this patch, layered directors needed to be destroyed top to
bottom, and whenever that order was missed, we would panic, because a
to-be-destroyed director still had references to it.

One special case where this issue would always trigger are looped
directors. Those should not be used and will cause havoc, which is a
separate issue #3899. But we should still be able to unconfigure such
a configuration.

We solve the destruction order issue by making it a two step process:

When a director is destroyed through VRT_DelDirector, a new release
function is called, which has to disassociate any backends. The
director then loses a reference, and when all references are gone, the
destroy function is called.

The new callback would not be necessary for the cases in varnish-cache
today, directors could simply disassociate any backends before calling
VRT_DelDirector. But this would complicate or even make impossible
transfer of director ownership, where the code responsible for
creating a director is not the same as the one calling
VRT_DelDirector(). As a side effect, it also helps clarity.

Fixes #3895