code: def since(self, ts):
          
          spec = {'ts': {'$gt': ts}}
          cursor = self.query(spec)
          while True:
              # todo: trap InvalidDocument errors:
              # except bson.errors.InvalidDocument as e:
              #  logging.info(repr(e))
              for doc in cursor:
                  yield doc
              if not cursor.alive:
                  break
              time.sleep(1)
docs: Query the oplog for items since ts and then return