parseStoreFileName/makeStoreFileName() takes a prefix string.

Instead of taking an int 'idx', the functions are now generified to
take a string prefix, allowing resued for view index files.
Refactored out latestStoreFileName() helper func.

For re-use for view index files.
Handle views compaction and restart better.

If the server restarts, look for the latest compacted views file,
not the 0th one.