sync tarfile stream diligently when writing new objects

sync tarfile stream diligently when writing new objects

Turns out all the offsets written to the index when neither
encrypting nor compressing were, well, … off. In fact they would
only be updated at tar block boundaries due to buffering. Since
“last_block_offset” record keeping blatantly violates layering
boundaries, it would only work reliably with the concat
compression and encryption modes that do the same.

Sync when adding a new object so we get the accurate offset
value. Voilà, recovery now works with uncompressed and
unencrypted archives as well