Perform checks for unindexed files and stale index entries on startup

Summary:
Right now, if a file was moved / removed while baloo_file was not running
(a common use-case for those who have dualboot), there is no way baloo will
become aware of such change (apart from running balooctl check by hand).

There will be stale (if a file was removed) or invalid (pointing to non-existing file,
if a file was moved) index entries, they will pop up if user performs search, which is
frustrating. I suggest performing those checks on baloo_file startup.

Test Plan:

1. balooctl stop && echo "hello world" > ~/new-file && balooctl start
2. after some time: balooshow ~/new-file, and see if it's indexed

Reviewers: Baloo, Frameworks, vhanda, ngraham

Reviewed By: Baloo, ngraham

Subscribers: bruns, ngraham, kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D15939