[PATCH] Support aborting the atomic context

[Floris Bruynooghe]

This is an implementation of what was suggested in
id:87v9k96xtl.fsf at powell.devork.be It closes the database as that is
the only safe way to do this afaik.

Currently when the database is closed there are still a bunch of
operations which can result in segfaults.  Yet the API also promises
that some operations are still valid, basically those which only
access already previously retrieved information.  It would be nice to
find a good solution for this in the python bindings, but I don't yet
know what this would be.