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 preceding 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.

The implementation will try to preallocate space for the restored files
on the filesystem to prevent file fragmentation. This ensures good read
performance for large files, like for example VM images. If preallocating
space is not supported by the filesystem, then this step is silently skipped.

https://github.com/restic/restic/pull/2195
https://github.com/restic/restic/pull/2893
