go/zodb: Prepare to rework/fix LiveCache control

We are going to rework LiveCache to support both retaining objects in
cache even though if all pointers to them went away (currently such
objects disappear from cache), and to tell the cache that such and such
object should not be cached at all, e.g. not to evict all other objects
on one-time large reads.

This will be done by LiveCacheControl giving cache policy classification
for an object. Prepare for that and change

	WantEvict(obj) -> bool

to

	PCacheClassify(obj) PCachePolicy

which can give more information to LiveCache about how to handle an
object.

This is only a preparatory patch as there is neither no new
functionality nor fixes here.