- Add some expository comments.

- Add some asserts documenting non-obvious assumptions.

- Make modifiedInVersion a delegated method, since it does nothing
  special.

- Fix a bug in undo(); it was passing a list of triples (oid,None,'')
  to the server.invalidate() call rather than a list of tuples
  (oid,'').  There are no unit tests for undo(), so this was never
  caught.  I guess few people care about undo() now that we have
  transactionalUndo().

- In ImmediateCommitStrategy.store(), only catch exceptions that
  inherit from Exception raised by the storage's store() call; the
  client is only checking for those in the list of serials.

- Remove unused instance variable 'invalidated' from the
  DelayedCommitStrategy class.