Maven remote index migration and refactoring

[mbien]

refactoring: split NexusRepositoryIndexerImpl to make it more maintainable

• moves about half of the code into a dedicated class
• queries moved to NexusRepositoryQueries
• index management remains in NexusRepositoryIndexManager

automatically move remote maven index from old into current cache, if:

• both cache folders share the same parent
• the cache folder is from an older release
• the index itself is still somewhat up to date
• current NB instance is not a dev build

note:

-J-Dmaven.indexing.diag.release=25 can be used to simulate a release version. This would move the index from <25 into the cache for current if above conditions apply (dev builds should not move anything). The index can be simply moved back by hand after testing.

This was planned for a while and prepared in past by adding "lucene backwards codecs" but I never got to implementing it. Was also mentioned in #6365 (and other places).

[matthiasblaesing]

Only eyeballed this, but looks good to me. Please consider to cleanup whitespace before merge. If you run "Search and Replace" with Regexp enabled and using the search pattern [ \t]+$ you see what I mean (I noticed that when diffing the two new files with the original implementation)

[mbien]

good catch, not sure where this was from. I cleaned up NexusRepositoryQueries completely (since its a new file) and some spots in NexusRepositoryIndexManager.

[matthiasblaesing]

Thank you.

[mbien]

btw I forgot to mention that the main reason I extracted the queries is because this would make it a little bit easier to drop the remote index code if sonatype decides to discontinue their index - the local bits could be kept and selective queries reimplemented which call a service or use maven plugins which fetches metadata directly to the local repo.

In any case the one class did too much at once. Thanks for reviewing!