For memory and sled, it's trivial, as we already have a Cursor<Vec<u8>>.
For gRPC, we simply reject going backwards, and skip n bytes for now.
Once the gRPC protocol gets support for offsets and verified streaming,
this can be improved.
Change-Id: I734066a514aed287ea3db64bfb1680911ac1eeb0
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8885
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>
