Run one step of this algorithm:
        * find the set of missing zonefiles
        * try to fetch each of them
        * store them
        * update our zonefile database

        Fetch rarest zonefiles first, but batch
        whenever possible.

        Return the number of zonefiles fetched

def step(self, path=None, peer_table=None):
        """
        Run one step of this algorithm:
        * find the set of missing zonefiles
        * try to fetch each of them
        * store them
        * update our zonefile database

        Fetch rarest zonefiles first, but batch
        whenever possible.

        Return the number of zonefiles fetched
        """

        # if os.environ.get("BLOCKSTACK_TEST", None) == "1":
        #    log.debug("%s: %s step" % (self.hostport, self.__class__.__name__))

        if path is None:
            path = self.atlasdb_path

        num_fetched = 0
        missing_zinfo = None
        peer_hostports = None

        with AtlasPeerTableLocked(peer_table) as ptbl: 
            missing_zfinfo = atlas_find_missing_zonefile_availability( peer_table=ptbl, path=path )
            peer_hostports = ptbl.keys()[:]

        # ask for zonefiles in rarest-first order
        zonefile_ranking = [ (missing_zfinfo[zfhash]['popularity'], zfhash) for zfhash in missing_zfinfo.keys() ]
        zonefile_ranking.sort()
        zonefile_hashes = list(set([zfhash for (_, zfhash) in zonefile_ranking]))
        zonefile_names = dict([(zfhash, missing_zfinfo[zfhash]['names']) for zfhash in zonefile_hashes])
        zonefile_txids = dict([(zfhash, missing_zfinfo[zfhash]['txid']) for zfhash in zonefile_hashes])
        zonefile_block_heights = dict([(zfhash, missing_zfinfo[zfhash]['block_heights']) for zfhash in zonefile_hashes])
        zonefile_origins = self.find_zonefile_origins( missing_zfinfo, peer_hostports )

        # filter out the ones that are already cached
        for i in xrange(0, len(zonefile_hashes)):
            # is this zonefile already cached?
            zfhash = zonefile_hashes[i]
            present = is_zonefile_cached( zfhash, self.zonefile_dir, validate=True )
            if present:
                log.debug("%s: zonefile %s already cached.  Marking present" % (self.hostport, zfhash))
                zonefile_hashes[i] = None

                # mark it as present
                self.set_zonefile_present(zfhash, min(zonefile_block_heights[zfhash]), path=path)

        zonefile_hashes = filter( lambda zfh: zfh is not None, zonefile_hashes )

        if len(zonefile_hashes) > 0:
            log.debug("%s: missing %s unique zonefiles" % (self.hostport, len(zonefile_hashes)))
        
        while len(zonefile_hashes) > 0 and self.running:

            zfhash = zonefile_hashes[0]
            zfnames = zonefile_names[zfhash]
            zftxid = zonefile_txids[zfhash]
            peers = missing_zfinfo[zfhash]['peers']
           
            # try this zonefile's hosts in order by perceived availability
            peers = atlas_rank_peers_by_health( peer_list=peers, with_zero_requests=True )
            if len(peers) > 0:
                log.debug("%s: zonefile %s available from %s peers (%s...)" % (self.hostport, zfhash, len(peers), ",".join(peers[:min(5, len(peers))])))

            for peer_hostport in peers:

                if zfhash not in zonefile_origins[peer_hostport]:
                    # not available
                    log.debug("%s not available from %s" % (zfhash, peer_hostport))
                    continue

                # what other zonefiles can we get?
                # only ask for the ones we don't have
                peer_zonefile_hashes = []
                for zfh in zonefile_origins[peer_hostport]:
                    if zfh in zonefile_hashes:
                        # can ask for this one too
                        peer_zonefile_hashes.append( zfh )

                if len(peer_zonefile_hashes) == 0:
                    log.debug("%s: No zonefiles available from %s" % (self.hostport, peer_hostport))
                    continue

                # get them all
                log.debug("%s: get %s zonefiles from %s" % (self.hostport, len(peer_zonefile_hashes), peer_hostport))
                zonefiles = atlas_get_zonefiles( self.hostport, peer_hostport, peer_zonefile_hashes, peer_table=peer_table )
                if zonefiles is not None:

                    # got zonefiles!
                    stored_zfhashes = self.store_zonefiles( zonefile_names, zonefiles, zonefile_txids, zonefile_block_heights, peer_zonefile_hashes, peer_hostport, path )
                    
                    # don't ask again
                    log.debug("Stored %s zonefiles" % len(stored_zfhashes))
                    for zfh in stored_zfhashes:
                        if zfh in peer_zonefile_hashes:
                            peer_zonefile_hashes.remove(zfh)
                        if zfh in zonefile_hashes:
                            zonefile_hashes.remove(zfh)

                        num_fetched += 1
                
                else:
                    log.debug("%s: no data received from %s" % (self.hostport, peer_hostport))

                with AtlasPeerTableLocked() as ptbl:
                    # if the node didn't actually have these zonefiles, then 
                    # update their inventories so we don't ask for them again.
                    # TODO: do NOT ban nodes that repeatedly lie to us, since the "node" could be a load-balancer for a set of nodes that may or may not have the zonefile
                    for zfh in peer_zonefile_hashes:
                        log.debug("%s: %s did not have %s" % (self.hostport, peer_hostport, zfh))
                        atlas_peer_set_zonefile_status( peer_hostport, zfh, False, zonefile_bits=missing_zfinfo[zfh]['indexes'], peer_table=ptbl )

                        if zfh in zonefile_origins[peer_hostport]:
                            zonefile_origins[peer_hostport].remove( zfh )


            # done with this zonefile
            if zfhash in zonefile_hashes:
                zonefile_hashes.remove(zfhash)

        if len(zonefile_hashes) > 0 or num_fetched > 0:
            log.debug("%s: fetched %s zonefiles" % (self.hostport, num_fetched))

        return num_fetched