until a bucket is garbage collected (which may be much later than the
  expiration of that bucket), we "finalize" a bucket as soon as possible
  after it gets expired.  This effectively means that the "delete notifier"
  will be called much closer to the time that a transient object actually
  expires.
                                                                                
- Add a "_last_finalized_timeslice" counter; this counter keeps track of 
  the bucket which was finalized last.  Set the initial value of
  "_last_finalized_timeslice" to -period; this services the unit tests for
  finalization, where there can actually be a timeslice that is 0 and we 
   need to finalize that bucket.
                                                                                
- Add a series of locks to prevent finalization, replentishment, and
  garbage collection from being attempted by more than one thread
  simultaneously.
                                                                                
- Add "Fake" module for interactive testing purposes (swap out BTree for
  simpler object during stress tests for isolation purposes).
                                                                                
- Allow DATA_CLASS to be specified (this is the main data structure
  class).

- Update docs and tests to account for new finalization strategy.