[CBM] The effect of synchronous backfill are not seen with PiTR enabled backups

What's the issue?
First of all, synchronous backfill was introduced for backup to S3 to avoid streaming all 1024 vBuckets at once (but retain similar/the same consistency guarantees).

Second, PiTR doesn't stream all snapshots in a single DCP stream; we re-request the next DCP stream for each PiTR snapshot.

Streaming each snapshot separately will negate the benefits of synchronous backfill because all 1024 vBuckets will be streaming at once again.

This causes a couple of major problems:
1) We lose the performance benefit of synchronous backfill when using PiTR
2) We lose the memory benefit for backup to S3 (the reason we initially added synchronous backfill)
3) It means the progress bar is broken for PiTR aware backups