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.

openraft/src/raft/mod.rs

@@ -318,7 +318,7 @@ where C: RaftTypeConfig
             rx_metrics,
             rx_data_metrics,
             rx_server_metrics,
-            tx_shutdown: Mutex::new(Some(tx_shutdown)),
+            tx_shutdown: std::sync::Mutex::new(Some(tx_shutdown)),
             core_state: Mutex::new(CoreState::Running(core_handle)),
 
             snapshot: Mutex::new(None),
@@ -919,7 +919,7 @@ where C: RaftTypeConfig
     ///
     /// It sends a shutdown signal and waits until `RaftCore` returns.
     pub async fn shutdown(&self) -> Result<(), JoinErrorOf<C>> {
-        if let Some(tx) = self.inner.tx_shutdown.lock().await.take() {
+        if let Some(tx) = self.inner.tx_shutdown.lock().unwrap().take() {
             // A failure to send means the RaftCore is already shutdown. Continue to check the task
             // return value.
             let send_res = tx.send(());

openraft/src/raft/raft_inner.rs

@@ -40,9 +40,7 @@ where C: RaftTypeConfig
     pub(in crate::raft) rx_data_metrics: WatchReceiverOf<C, RaftDataMetrics<C>>,
     pub(in crate::raft) rx_server_metrics: WatchReceiverOf<C, RaftServerMetrics<C>>,
 
-    // TODO(xp): it does not need to be a async mutex.
-    #[allow(clippy::type_complexity)]
-    pub(in crate::raft) tx_shutdown: Mutex<Option<OneshotSenderOf<C, ()>>>,
+    pub(in crate::raft) tx_shutdown: std::sync::Mutex<Option<OneshotSenderOf<C, ()>>>,
     pub(in crate::raft) core_state: Mutex<CoreState<C>>,
 
     /// The ongoing snapshot transmission.