- 'singlethreaded' compile-time feature gate.

The new feature gate forces the raft instance to be used by a single thread by not implementing `Send` for certain data structures and substituting calls to `tokio::spawn` with `tokio::spawn_local` when using the `tokio` asynchronous runtime.

- Fix: [datafuselabs#862](datafuselabs#862)

**Checklist**

- [x] Updated guide with pertinent info (may not always apply). <!-- Mark complete if nothing to do. -->
- [o] Squash down commits to one or two logical commits which clearly describe the work you've done.
- [o] Unittest is a friend:)