This adds the simpler ChunkService trait, and an implementation for it
using sled, and one using a HashMap.
Change-Id: Icb0fdc41b37b44e9e9e4f548d0f4acae1d83b71e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8086
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Tested-by: BuildkiteCI
