Enhancement: Reduce memory usage by up to 25%

The in-memory index has been optimized to be more garbage collection friendly.
Restic now defaults to `GOGC=50` to run the Go garbage collector more
frequently.

https://github.com/restic/restic/issues/3328
https://github.com/restic/restic/pull/4352
https://github.com/restic/restic/pull/4353
