Enhancement: Concurrent restore

This change significantly improves restore performance, especially
when using high-latency remote repositories like B2. 

Implementation uses threads to download and process miltiple remote 
files concurrently. To further reduce restore time, each remote
file is downloaded using single repository request.

Old restore implementation can be enabled with `--signethreaded` flag.
Use `--verify` restore flag to read restored files and verify their
content checksum.

https://github.com/restic/restic/issues/1605
https://github.com/restic/restic/pull/1719