Add default ackTimeout(30s) for dead letter policy (apache#3014)

Fix issue apache#2987 
### Motivation

In version 2.2.0, support DeadLetterTopic feature. This feature based on message redelivery. So ack timeout is necessary.

### Modifications

Set ackTimeout(30s) when enable the dead letter policy but not set the ackTimeout;

pulsar-client/src/main/java/org/apache/pulsar/client/api/ConsumerBuilder.java

@@ -365,6 +365,8 @@ public interface ConsumerBuilder<T> extends Cloneable {
      *          .deadLetterPolicy(DeadLetterPolicy.builder().maxRedeliverCount(10).deadLetterTopic("your-topic-name").build())
      *          .subscribe();
      * </pre>
+     * When a dead letter policy is specified, and no ackTimeoutMillis is specified,
+     * then the ack timeout will be set to 30000 millisecond
      */
     ConsumerBuilder<T> deadLetterPolicy(DeadLetterPolicy deadLetterPolicy);
 }

pulsar-client/src/main/java/org/apache/pulsar/client/impl/ConsumerBuilderImpl.java

@@ -60,6 +60,8 @@ public class ConsumerBuilderImpl<T> implements ConsumerBuilder<T> {
     private List<ConsumerInterceptor<T>> interceptorList;
 
     private static long MIN_ACK_TIMEOUT_MILLIS = 1000;
+    private static long DEFAULT_ACK_TIMEOUT_MILLIS_FOR_DEAD_LETTER = 30000L;
+
 
     public ConsumerBuilderImpl(PulsarClientImpl client, Schema<T> schema) {
         this(client, new ConsumerConfigurationData<T>(), schema);
@@ -266,7 +268,12 @@ public ConsumerBuilder<T> intercept(ConsumerInterceptor<T>... interceptors) {
 
     @Override
     public ConsumerBuilder<T> deadLetterPolicy(DeadLetterPolicy deadLetterPolicy) {
-        conf.setDeadLetterPolicy(deadLetterPolicy);
+        if (deadLetterPolicy != null) {
+            if (conf.getAckTimeoutMillis() == 0) {
+                conf.setAckTimeoutMillis(DEFAULT_ACK_TIMEOUT_MILLIS_FOR_DEAD_LETTER);
+            }
+            conf.setDeadLetterPolicy(deadLetterPolicy);
+        }
         return this;
     }