NEXUS-6187: Detect updated files in storage

An imperfect solution, but might work in scenario where
deployed/overwritten files has timestamps that differ
more than a millisecond.

Still, this solution is just as bad as "recreating"
attributes in general are, all the custom ones are
lost forever.

This change is only a band-aid, or at least
should be considered as such.

Issue
https://issues.sonatype.org/browse/NEXUS-6187