s3d only sends full ~40 MB blocks to Sia, so a handful of small files can
stay in the local buffer forever (seen: 63 objects, 6 MiB, nothing on Sia).
An opt-in setting now flushes once the pending set has not changed for N
minutes, judged from s3d's upload stats so uploads from any S3 client count.
Failed flushes back off up to an hour instead of hammering a broken indexer.
