thread, instead of doing it every nth ZODB transaction.  We can
actually provide a base class for both the checkpointing and
autopacking threads here.  Changes here include:

SLEEP_TIME: how often (in seconds) a background thread should wake up
to see if there's work to do.

True, False: Add these for older Pythons.

PackStop: New class that acts as an escape hatch for pack operations.

BerkeleyConfig:
    - change interval from 100 to 0.  Now that this controls the
      checkpointing thread, the current default is to not spawn the
      thread.  I'm probably going to change this once I figure out
      what a good value is.

BerkeleyBase:
    - __init__(): Start the checkpointing thread if interval > 0

    - _setupDB(): Add some additional keyword args so that the QUEUE
      style tables can use this convenience method too.

    - close(): Be sure to stop and join the checkpointing thread

    - _docheckpoint(): Removed

    - _withtxn(): Catch PackStop escape hatch exceptions.  This one
      aborts the current Berkeley transaction but eats the exception.
      Also, don't call _docheckpoint() here.

     - docheckpoint(): New method which the checkpointing threads can
       call.

env_from_string(): use DB_RECOVER_FATAL for autorecovery on open.

_WorkThread: New base class for the checkpointing and autopacking
threads.

_CheckPoint: The common checkpointing thread class.