A log whose generation isn't its snapshot's was skipped as a leftover of a
compaction cut short, but neither generation is under a checksum: one flipped
bit made the server serve the older document, report nothing and overwrite the
log. Replay such a log over the snapshot instead: replay skips what the
snapshot already holds, so a genuine leftover changes nothing, while edits the
snapshot lacks are recovered and the damage is reported.
