Enhancement: Improve `prune` performance and make it more customizable

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

Restic used to rebuild the index from scratch after pruning. This could lead
to missing packs in the index in some cases for eventually consistent backends
such as e.g. AWS S3. This behavior is now changed and the index rebuilding
uses the information already known by `prune`.

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

Moreover, `prune` now accepts the `--dry-run` option and also running
`forget --dry-run --prune` will show what `prune` would do.

This enhancement also fixes several open issues, e.g.:
- https://github.com/restic/restic/issues/1140
- https://github.com/restic/restic/issues/1599
- 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
https://github.com/restic/restic/pull/2842