SecondsToTTL truncates to the volume TTL grid, so 3599 remaining
seconds became 59m and anything under a minute became no TTL at all -
permanent chunks under an expiring entry. Round the remaining lifetime
up to the smallest representable value instead, and anchor it the way
FindEntry expires entries: S3-expiring entries age from Mtime, others
from Crtime, so a recently overwritten S3 object is no longer treated
as nearly expired.
Also bind each layout to a digest of the chunk list it described.
Offset writes and appends keep Extended while changing the chunks, so
a same-size partial write used to leave the old playlist and extents
being served over new bytes; the views now detect the mismatch and
answer 404 until the file is re-ingested or repacked.
Chris Lu
10b64686ba