Entry point for indexing:
        * scan the blockchain from start_block to end_block and make sure we're up-to-date
        * process any newly-arrived zone files and re-index the affected subdomains

def index(self, block_start, block_end):
        """
        Entry point for indexing:
        * scan the blockchain from start_block to end_block and make sure we're up-to-date
        * process any newly-arrived zone files and re-index the affected subdomains
        """
        log.debug("BEGIN Processing zonefiles discovered since last re-indexing")
        t1 = time.time()
        self.index_discovered_zonefiles(block_end)
        t2 = time.time()
        log.debug("END Processing zonefiles discovered since last re-indexing ({} seconds)".format(t2 - t1))