by properly cleaning up the busyobj

Also move the relevant code from cache_vrt.c to cache_fetch.c

As we fini the director during cleanup, we now also need to handle the
backend connection gone missing in vbf_stp_fetch(). The hypothetical
alternative would be to not fini the director, but I believe this is not
safe in case it also used some workspace.

Fixes #3009

 Conflicts:
	bin/varnishd/cache/cache_fetch.c
	bin/varnishd/cache/cache_varnishd.h
	bin/varnishd/cache/cache_vrt.c