Stop processing read requests at shutdown.

kvraft/src/server.rs

@@ -270,6 +270,10 @@ impl KVServer {
         &self,
         key: String,
     ) -> Result<Option<String>, KVError> {
+        if !self.keep_running.load(Ordering::SeqCst) {
+            return Err(KVError::NotLeader);
+        }
+
         let result_fut = match self.rf.verify_authority_async() {
             Some(result_fut) => result_fut,
             None => return Err(KVError::NotLeader),
@@ -499,6 +503,8 @@ impl KVServer {
         for (_, (_, sender)) in self.state.lock().queries.drain() {
             let _ = sender.send(Err(CommitError::NotLeader));
         }
+        // Drop all read requests.
+        self.state.lock().index_subscribers.drain();
 
         let rf = self.raft().clone();
         // We must drop self to remove the only clone of raft, so that