remaining problem with full Berkeley storage's transactionalUndo().
Specifically:

    tid 1: create object 1
    tid 2: modify object 1
    tid 3: undo tid 2
    ----------------------
    tid 4: undo tid 1 (1st alternative universe)
    ----------------------
    tid 4: undo tid 3 (2nd alternative universe)

The new tests are checkUndoCreationBranch1() and
checkUndoCreationBranch2(), corresponding to alternative universes 1
and 2, respectively.  Full storage currently fails a.u. #2.
FileStorage passes.