Refactor: Replace async Mutex for RaftInner.core_state with standard Mutex and a watch channel

[drmingdrmer]

Changelog

Refactor: Replace async Mutex for RaftInner.core_state with standard Mutex and a watch channel

In this commit, when joining the RaftCore task, the core_state is
first switched from Running to Joining. The thread then blocks
while awaiting the completion of the RaftCore task. Any other threads
observing the Joining state will wait for the first thread to finish
by monitoring a watch channel created by the initial thread.

Refactor: RaftInner::tx_shutdown changed to use std Mutex

RaftInner::tx_shutdown has been changed to use a standard
(synchronous) Mutex instead of an asynchronous one. This change is made
because tx_shutdown does not span across .await points, making the
asynchronous capabilities unnecessary.

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This change is 

[schreter]

Reviewed 3 of 3 files at r1, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @drmingdrmer)

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openraft/src/raft/core_state.rs line 14 at r1 (raw file):

    Running(JoinHandleOf<C, Result<Infallible, Fatal<C>>>),

    Joining(WatchReceiverOf<C, bool>),

Maybe add a doc comment?

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openraft/src/raft/raft_inner.rs line 227 at r1 (raw file):

            Err(mut rx) => {
                // Other thread is waiting for the core to finish.
                let _ = rx.changed().await;

Though this should be sufficient (channel can be cloned only while it's definitely set to false), don't we need to wait for update to true instead just for the change? What about spurious wakeups?