Enhancement: Improve pruning performance and make pruning more customizable

The prune command is now much faster. This is especially the case for remote
repositories or repositories with not much data to prune.
Also the memory usage of the prune command is now reduced.

By default the prune command now no longer removes all unused blobs. This
behavior can be fine-tuned by new options, like tolerated unused space or 
maximum size of packs to repack. For more details, see
https://restic.readthedocs.io/en/stable/060_forget.html

Moreover, prune now accepts the dry-run option and forget --dry-run --prune 
also shows what prune would do.

Fixes several open issues, e.g.:
https://github.com/restic/restic/issues/1140
https://github.com/restic/restic/issues/1985
https://github.com/restic/restic/issues/2112
https://github.com/restic/restic/issues/2227
https://github.com/restic/restic/issues/2305

https://github.com/restic/restic/pull/2718