The DB's invalidate() method takes a set of oids corresponding to all
the changes from a data manager for one transaction.  All the objects
are invalidated at once.

Add a few tests in testZODB of the new code.  The tests just cover
corner cases, because I can't think of a sensible way to test the
atomicity.  When it has failed in the past, it's been caused by
nearly-impossible to reproduce data races.

This fix needs to be backported to Zope 2.6, but only after assessing
how significant an impact the API change will have.