Removed undo and version support.

Removed close, cleanup, and pack. Storages should implement these and
providing stubs encourages broken implementations.

history should return a sequence.

Renamed getSerial to getTid.

Added a ZEO-required storage tpc_transaction method to get a storage's
current transaction, if any.

Reduced loadBefore to an empty method.  This produces the same output
as before. :)  There was an elaborate implementation that only worked
with an implementation that didn't need it. :)

Removed getExtensionMethods, which is not part of the storage API.

Created a copy *function* from copyTransactionsFrom. Now
copyTransactionsFrom just uses that.  Now this logic, such as it is,
can be reused without inheritence.