TrackerMinerFS: Fix mtime cache for crawl directories themselves.

Mtime cache works by querying the parent folder, but for crawl directories
there wouldn't be any data for its parent folder, needlessly reindexing
these again, and leaving orphaned urns in the database. This problem would
add up in consecutive runs, as direct children nfo:belongsToContainer would
still point to the orphaned node, thus spreading the problem through the
hierarchy.