The picklelog branch attempts to improve performance and reduce the
possibility of lock file exhaustion by optimistically writing pickle
and metadata tables at the store() call instead of during the
_finish() call.  store() will be bounded in the number of objects it
touches, but _finish() is unbounded, so we try to reduce the number of
database pages the latter might actually touch.

Other performance improvements are implemented based on hotshot
profiling.  The use of an extension module for Python 2.2 also speeds
up a critical loop.

This also implements the periodic checkpointing to improve recovery
times when the database is not cleanly shutdown.