tpc_finish(); i.e. it acquires the lock, checks the argument, and then
calls self._vote().  Note that the transaction passed to tpc_vote()
isn't passed to self._vote(), since I don't what would be useful.

BaseStorage._vote(): current no-op.

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.

versionEmpty.

in a version, we fall back to non-version data.

Undo wasn't tested properly. The tests didn't get the undo ids
from the transaction log.

recognize other valid failure modes.

object hasn't been modified in a version, we fall back to non-version
data.

There are a couple of places where the spec was unclear and I punted
for now.

To do this, I had to change the temprary index to be a mapping
object (dictionary) rather than a sequence (list).

the fact that tpc_vote and tpc_finish can be called extra times.

Also added code to same test to undo the transaction that undid
multiple transaction to exercise bug found in FileStorage.

We now pass the tests we test. We still have to add the version tests,
which we were failing before.

undone and then redone, if the underlying storage supports
transactionalUndo.

Also fixed a bug in an unused method. The bug causes warnings in
Python 2.1.

Unfortutely, tpc_finish didn't handle this case correctly.

committed state in. This is needed wheh the "committed" state,
um, hasn't been committed yet, such as when you are undoing
multiple transactions transactionally.

made a true mixin (not a subclass).

(it's a true mixin).

unittest.TestCase.  It also provides the setUp(), _close(), and
tearDown() methods, as well as the _dostore() convenience method.

_dostore(), setUp(), _close(), tearDown(): Moved out of this class and
into StorageTestBase.

Removed some log-ago-commented-out code.

Added some doc strings.

Added a bunch of not-yet-enabled (because not yet fully working)
transactional undo code.

to a directory other than the ZEO package directory.

that allow start.py to recover.

Thanks for Chris Withers for reminding about this.

rely on the ZEO __init__, which relies on fap, to sort things out.