Domain priority

This patch makes CassandraDatastore.ClaimNewHost honor the priority in domain_info when finding new hosts to crawl.
I think a comment explaining what this is doing may be helpful. Took me a while to get that it's using the priority given in the domain to make sure that we pulled all the hosts off ClaimNewHost in priority order (though of course it makes sense since priority is what we're testing). Part of it may be that Sscanf calls can be pretty cryptic. I'm alright with it if we provide a bit of clarity though.