restorer: Allow writing target file blobs out of order

Much simpler implementation that guarantees each required pack
is downloaded only once (and hence does not need to manage
pack cache). Also improves large file restore performance.

Signed-off-by: Igor Fedorenko <igor@ifedorenko.com>

changelog/unreleased/pull-2195

@@ -0,0 +1,17 @@
+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