Refactor of PersistenceEngineFixture where it purges on dispose by default. Remove ability to override this until it's necesary. To support this added IsDisposed property to IPersistStreams so a purge isn't attempted on a disposed resource.
Remove obsolete field set.
Remove obsolete comment
Rounds down the start datetime in GetFrom and GetFromTo to the nearest second in SqlPersistenceEngine to accomodate differences in DateTime precisions in variours SQL engines.