Enhancement: Simplify and improve restore performance

Significantly improves restore performance of large files (i.e. 50M+):
https://github.com/restic/restic/issues/2074
https://forum.restic.net/t/restore-using-rclone-gdrive-backend-is-slow/1112/8
https://forum.restic.net/t/degraded-restore-performance-s3-backend/1400

Fixes "not enough cache capacity" error during restore:
https://github.com/restic/restic/issues/2244

NOTE: This new implementation does not guarantee order in which blobs
are written to the target files and, for example, the last blob of a
file can be written to the file before any of the preceeding file blobs.
It is therefore possible to have gaps in the data written to the target
files if restore fails or interrupted by the user.

https://github.com/restic/restic/pull/2195