without circular references we do not need weak goal pointers except for
caches, which should not prevent goal destructors running. caches though
cannot create circular references even when they keep strong references.
if we removed goals from caches when their work() is fully finished, not
when their destructors are run, we could keep strong pointers in caches.
since we do not gain much from this we keep those pointers weak for now.
Change-Id: I1d4a6850ff5e264443c90eb4531da89f5e97a3a0
eldritch horrors
a5240b23ab