lastUploadedCheckSum in FileIndex is only being updated for v0 files

[crspybits]

I just looked at this now, and for vN (mutable file) this FileIndex value doesn't get updated. Though, for now, I'm relying on this in isDuplicateV0Upload. In general, I need a way to test for v0 retries-- i.e., isDuplicateV0Upload. See SyncServerII/Neebla#25 (comment).

If I start using lastUploadedCheckSum more generally, which I should I should likely keep two check sums:

1. a v0 checksum and
2. a current vN checksum.

Though that could get confusing because I'm currently allowing (at least for relatively brief intervals) downloads of various (stale) file versions.