DB query system should not refute external read results

ExternalReadEntry implementation in the DB does not have a custom refute implementation which allows the query system to start refuting external read results.

This does not seem like the correct thing to do because the by nature ExternalReads are such that the query system knows nothing about their internals and the way their results get updated. Therefore it is not for the query system to say that an ExternalRead's results are to be considered refuted.