Rust: Project 2 "Part 6: Stateless vs. stateful KvStore" makes me confused.

[liufuyang]

https://github.com/pingcap/talent-plan/blob/master/rust/projects/project-2/project.md#part-6-stateless-vs-stateful-kvstore

Borrow Brian's comments from the slack channel: "The statefulness is useless in project 2 where there is no server, but is important in project 3. The text should clarify this, or the task moved to project 3"

@brson