def get(self, now):
raise NotImplementedError()<EOL>
Get a bucket key to compact.  If none are available, returns
None.

:param now: The current time, as a float.  Used to ensure the
            bucket key has been aged sufficiently to be
            quiescent.

:returns: A bucket key ready for compaction, or None if no
          bucket keys are available or none have aged
          sufficiently.