adjust unmerged backfill lfn

Trigger for this change is to write unmerged backfill to
/store/unmerged instead of /store/backfill. Take this
opportunity to adjust the LFN configuration, locking it
down. Instead of configuring the LFN prefix, now only
can specify if no backfill or backfill 1 or 2 are used.
Merge pull request #4086 from hufnagel/backfill-lfn

adjust unmerged backfill lfn