libtracker-miner: Add envvar toggle to force directory updated checks

The bugs covered by the last patches might leave leftovers in the DB if
tracker-miner-fs updated a directory after it changed between runs, but
wouldn't delete contents that were removed from the filesystem.

Add this toggle envvar to point people to a solution, if you've seen errors
like "UNIQUE constraint failed: nie:DataObject.nie:url" in journald, you're
affected by these bugs, and should run tracker-miner-fs once with
TRACKER_MINER_FORCE_CHECK_UPDATED set in the environment. Running just
once will be enough, enabling this always will incur in startup performance
penalties.