Fix CPU 100% when deleting namespace (apache#10337)

### Motivation
When deleting the namespace, the namespace Policies will be marked as deleted.
This will trigger topic's `onPoliciesUpdate`
However, in onPoliciesUpdate, the data of the Policies node on zk will be read, such as: `checkReplicationAndRetryOnFailure`
Due to the deletion of the namespace, the zk node may no longer exist at this time. 
Failure to read data will trigger infinite retries.
https://github.com/apache/pulsar/blob/e970c2947aff9231202ab72bdbad047d85c55633/pulsar-broker/src/main/java/org/apache/pulsar/broker/service/persistent/PersistentTopic.java#L1175-L1193

If there are many topics, there will be a short-term CPU spike

![image](https://user-images.githubusercontent.com/9758905/115834541-ebc32480-a447-11eb-887a-95c4a3d1adf1.png)

pulsar-broker/src/main/java/org/apache/pulsar/broker/service/persistent/PersistentTopic.java

@@ -1172,7 +1172,8 @@ public void closeFailed(ManagedLedgerException exception, Object ctx) {
         return closeFuture;
     }
 
-    private CompletableFuture<Void> checkReplicationAndRetryOnFailure() {
+    @VisibleForTesting
+    CompletableFuture<Void> checkReplicationAndRetryOnFailure() {
         CompletableFuture<Void> result = new CompletableFuture<Void>();
         checkReplication().thenAccept(res -> {
             log.info("[{}] Policies updated successfully", topic);
@@ -2183,6 +2184,10 @@ public CompletableFuture<Void> onPoliciesUpdate(Policies data) {
             log.debug("[{}] isEncryptionRequired changes: {} -> {}", topic, isEncryptionRequired,
                     data.encryption_required);
         }
+        if (data.deleted) {
+            log.debug("Ignore the update because it has been deleted : {}", data);
+            return CompletableFuture.completedFuture(null);
+        }
         isEncryptionRequired = data.encryption_required;
 
         setSchemaCompatibilityStrategy(data);

pulsar-broker/src/test/java/org/apache/pulsar/broker/service/persistent/PersistentTopicTest.java

@@ -23,15 +23,18 @@
 import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.Mockito.doNothing;
 import static org.mockito.Mockito.mock;
-import static org.testng.Assert.assertEquals;
+import static org.mockito.Mockito.spy;
+import static org.mockito.Mockito.times;
+import static org.mockito.Mockito.verify;
 import static org.testng.Assert.assertFalse;
 import static org.testng.Assert.assertNotNull;
 import static org.testng.Assert.assertNull;
 
 import java.lang.reflect.Field;
+import java.util.UUID;
 import java.util.concurrent.TimeUnit;
-import java.util.concurrent.atomic.AtomicBoolean;
 
+import com.google.common.collect.Sets;
 import org.apache.bookkeeper.client.LedgerHandle;
 import org.apache.bookkeeper.mledger.ManagedLedger;
 import org.apache.pulsar.broker.service.BrokerTestBase;
@@ -43,6 +46,8 @@
 import org.apache.pulsar.client.api.SubscriptionType;
 import org.apache.pulsar.common.naming.NamespaceBundle;
 import org.apache.pulsar.common.naming.TopicName;
+import org.apache.pulsar.common.policies.data.Policies;
+import org.awaitility.Awaitility;
 import org.testng.annotations.AfterMethod;
 import org.testng.annotations.BeforeMethod;
 import org.testng.annotations.Test;
@@ -156,4 +161,30 @@ public void testUnblockStuckSubscription() throws Exception {
         msg = consumer2.receive(5, TimeUnit.SECONDS);
         assertNotNull(msg);
     }
+
+    @Test
+    public void testDeleteNamespaceInfiniteRetry() throws Exception {
+        //init namespace
+        final String myNamespace = "prop/ns" + UUID.randomUUID();
+        admin.namespaces().createNamespace(myNamespace, Sets.newHashSet("test"));
+        final String topic = "persistent://" + myNamespace + "/testDeleteNamespaceInfiniteRetry";
+        conf.setForceDeleteNamespaceAllowed(true);
+        //init topic and policies
+        pulsarClient.newProducer().topic(topic).create().close();
+        admin.namespaces().setMaxConsumersPerTopic(myNamespace, 0);
+        Awaitility.await().atMost(3, TimeUnit.SECONDS).until(()
+                -> admin.namespaces().getMaxConsumersPerTopic(myNamespace) == 0);
+
+        PersistentTopic persistentTopic =
+                spy((PersistentTopic) pulsar.getBrokerService().getTopicIfExists(topic).get().get());
+
+        Policies policies = new Policies();
+        policies.deleted = true;
+        persistentTopic.onPoliciesUpdate(policies);
+        verify(persistentTopic, times(0)).checkReplicationAndRetryOnFailure();
+
+        policies.deleted = false;
+        persistentTopic.onPoliciesUpdate(policies);
+        verify(persistentTopic, times(1)).checkReplicationAndRetryOnFailure();
+    }
 }