def get(self, now):
items = self.script(keys=[self.key], args=[now - self.min_age])<EOL>return items[<NUM_LIT:0>] if items else None<EOL>
Get a bucket key to compact.  If none are available, returns
None.  This uses a Lua script to ensure that the bucket key is
popped off the sorted set in an atomic fashion.

: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.