Don't mix const and non-const pointers. This should make jenkins happy again.
If workspace_thread is not be big enough to hold all the objcore pointers
that need to be purged we iterate until done.

Fixes #1551

Conflicts:
	bin/varnishd/cache/cache_hash.c
Ensure that we never call a VDP with a zero length unless we are done.

Fixes #1561
Make a dedicated cleanup function for req->vary_? to match the
dedicated setup function we have.

Fixes	#1553
Prepare for 4.0.2-rc1 release.