Fix concurrent modification exception. (#1493)

### Motivation

There is some flaky test when we stop the brokers.
```
Error:  Failures:
Error:    CacheInvalidatorTest.cleanup:111->KopProtocolHandlerTestBase.internalCleanup:319->KopProtocolHandlerTestBase.stopBroker:355->KopProtocolHandlerTestBase.stopBroker:351 » PulsarServer
Error:    TransactionWithOAuthBearerAuthTest.cleanup:72->TransactionTest.cleanup:77->KopProtocolHandlerTestBase.internalCleanup:319->KopProtocolHandlerTestBase.stopBroker:355->KopProtocolHandlerTestBase.stopBroker:351 » PulsarServer
```

The root cause is when we close the `TransactionMarkerChannelManager`,
we will traverse all entries of `handlerMap`,
but some of the entry already been removed,
we can use `ConcurrentHashMap` to avoid it.

### Modifications

Use `ConcurrentHashMap` instead of `HashMap`.

(cherry picked from commit 533efbd)

kafka-impl/src/main/java/io/streamnative/pulsar/handlers/kop/coordinator/transaction/TransactionMarkerChannelManager.java

@@ -27,7 +27,6 @@
 import java.io.IOException;
 import java.net.InetSocketAddress;
 import java.util.ArrayList;
-import java.util.HashMap;
 import java.util.HashSet;
 import java.util.List;
 import java.util.Map;
@@ -79,7 +78,8 @@ public class TransactionMarkerChannelManager {
 
     private final Bootstrap bootstrap;
 
-    private Map<InetSocketAddress, CompletableFuture<TransactionMarkerChannelHandler>> handlerMap = new HashMap<>();
+    private final Map<InetSocketAddress, CompletableFuture<TransactionMarkerChannelHandler>> handlerMap =
+            new ConcurrentHashMap<>();
 
     private TransactionStateManager txnStateManager;