ability to update an existing index without relying on timestamp

due to the possibility that timestamps can get out of sync, it should be possible to update an index without relying on these in a performant way (ie, not rebuilding from scratch).

for example, compare the artifact timestamp to the target location before deciding if an addition/update is needed and reading the JAR (possibly the slowest part of indexing - should confirm what % it takes)

this will also need to process deletions (which may not be handled in the normal indexing, depending on how that is implemented for discovery)