ensure that DB enum check is done only once per BF revision

When the server starts up, now `DBEnumCheck` now does *not* run if the server has the same Bio-Formats version as last time it started up.

--rebased-from #2406

In testing, it can help to slip into `dist/etc/logback.xml` a,
```xml
<logger name="ome.services.util" level="DEBUG"/>
```