* 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