* master:
  go/zodb: Fix TxnStatus constants type

Previously TxnComplete was typed, but TxnPacked and TxnInprogress were
untyped constants. Make all constants have TxnStatus type.

* master:
  go/zodb: Loader: Document that loaded buffer must not be modified
  go/zodb: Split Loader & co. interfaces from IStorage
  go/zodb: Document that Tid(0) is invalid while Oid(0) is valid.
  fixup! go/zodb: Tweak documentation a bit so it renders more well in godoc

The reason for this is that the buffer can be shared with other loaders
via cache.

There are situations when we want to work with, or emulate, only subset
of whole IStorage functionality, with one already established example
being Cache. For this reason split IStorage interface into finer-grained
ones. For now:

	- Loader
	- Prefetcher
	- Iterator
	- Committer
	- Notifier

Oid(0) represents root database object. We cannot change this.