Indexing service

[kirillt]

We should externalize the processing stages: indexing, metadata extraction, and previews generation. By externalization, we mean an external system entity should undertake these activities for each root folder, with the results subsequently pulled by the applications (Navigator, Shelf, Memo, etc.).

[kirillt]

Probably we can just protect index with new atomic files method from arklib.

This might cause excessive duplicates in index updates though. E.g. two apps update same index on same folder (one of them writes first, the other will write new version but big part of the update will be rejected because it's already written by the first app).