Generalization of raw access
As today the access to disc (the single file) is managed by a single struct (DiscRef), replace that with a trait that than would be implemented the current implementation and eventual new implementations, like:
- in memory only backed by a Vec that can be used for resiliency tests
- Specialized version for *nix systems with the use read_at and write_at that do not require lock
- Eventual raw device access bypassing the file system