Clean up the stale connections in all of the pools, and then
        get rid of empty pools.  Pools clean themselves every time a
        connection is fetched; this cleaning takes care of pools that
        aren't being used any more, so nothing is being gotten from
        them.

def clean(self):
        """
        Clean up the stale connections in all of the pools, and then
        get rid of empty pools.  Pools clean themselves every time a
        connection is fetched; this cleaning takes care of pools that
        aren't being used any more, so nothing is being gotten from
        them. 
        """
        with self.mutex:
            now = time.time()
            if self.last_clean_time + self.CLEAN_INTERVAL < now:
                to_remove = []
                for (host, pool) in self.host_to_pool.items():
                    pool.clean()
                    if pool.size() == 0:
                        to_remove.append(host)
                for host in to_remove:
                    del self.host_to_pool[host]
                self.last_clean_time = now