Enhancement: Improve backup speed with many small files

We have restructured the backup pipeline to continue reading files while all
upload connections are busy. This allows the backup to already prepare the next
data file such that the upload can continue right once the last one has
completed. This can especially improve the backup performance for high latency
backends.

The upload concurrency is now controlled using the `-o <backend-name>.connections=5`
option.

https://github.com/restic/restic/issues/2696
https://github.com/restic/restic/pull/3489
