set read_mode=majority when claiming a site to brozzle, to avoid weird thing where brozzler keeps claiming site it's already working on (not sure this is the cause of the problem but i don't see why else it might happen)

brozzler/frontier.py

@@ -91,7 +91,7 @@ class RethinkDbFrontier:
     def claim_site(self, worker_id):
         # XXX keep track of aggregate priority and prioritize sites accordingly?
         while True:
-            result = (self.r.table("sites")
+            result = (self.r.table("sites", read_mode="majority")
                     .between(
                         ["ACTIVE",rethinkdb.minval],
                         ["ACTIVE",rethinkdb.maxval],