Enhancement: Stream data in `check` and `prune` commands

The commands `check --read-data` and `prune` previously downloaded data files
into temporary files which could end up being written to disk. This could cause
a large amount of data being written to disk.

The pack files are now instead streamed, which removes the need for temporary
files. Please note that *uploads* during `backup` and `prune` still require
temporary files.

https://github.com/restic/restic/pull/3484
https://github.com/restic/restic/issues/3710
https://github.com/restic/restic/pull/3717
