'Freezes the timestamp used for resolve/delete database queries.
Frozen timestamp is used to consistently limit the datastore resolve and
delete queries by time range: from 0 to self.frozen_timestamp. This is
done to avoid possible race conditions, like accidentally deleting
notifications that were written by another process while we were
processing requests.'

def FreezeTimestamp(self):

    self.prev_frozen_timestamps.append(self.frozen_timestamp)
    self.frozen_timestamp = rdfvalue.RDFDatetime.Now()
