[FTS] moss stats on old files kept alive (ref-counted)

Idea that came up while discussing the recent moss store compaction performance results with Sundar Sridharan and Alex Gyryk...

Need a quick design writeup on a potential new, additional stat or two for moss store. Basically, how many "in-use" files are there, and how many actual files are there in the storage directory.

If the numbers aren't matching, or the numbers are too high, perhaps there's a reference count leak. Or, perhaps there are really slow queries or readers that are holding onto old files.