Enhancement: Parallelize scan of snapshot content in `copy` and `prune`

The `copy` and `prune` commands used to traverse the directories of
snapshots one by one to find used data. This snapshot traversal is
now parallelized which can speed up this step several times.

In addition the `check` command now reports how many snapshots have
already been processed.

https://github.com/restic/restic/pull/3106
