Enhancement: Reduce number of backend requests for rebuild-index and prune

We've found a way to reduce then number of backend requests for the
`rebuild-index` and `prune` operations. This significantly speeds up the
operations for high-latency backends.

https://github.com/restic/restic/issues/1567
https://github.com/restic/restic/pull/1574
https://github.com/restic/restic/pull/1575