[FTS] moss: Store.Stats() should emit FileInfo's for files

Hi Sundar,
There's some stats that moss provides, including a list of all the files that it thinks it has open, related to the Store.Stats() API, which invokes the store.go allFiles() function...

https://github.com/couchbase/moss/blob/master/store.go#L309

An improvement is that when allFiles() reads the directory and gathers each FileInfo, it might as well emit that info into the resulting map (e.g., the file's name, size, mode, modTime, isDir).

Doing this will allow us also to correlate when moss thinks it has a non-zero file-ref, but that file actually doesn't even exist on disk, related to issues like MB-24490 and to MB-24086.

Cheers,
steve