Reduce char[] creation on jvm heap. (apache#5055)

### Motivation

Use MoreObjects at toString() method will resulting in the creation of a large number of chars []:
![image](https://user-images.githubusercontent.com/12592133/63762972-61cf5b00-c8f6-11e9-8d5f-12b2f949b601.png)

Here is source of large number of calls to this method:

https://github.com/apache/pulsar/blob/master/pulsar-broker/src/main/java/org/apache/pulsar/broker/service/AbstractBaseDispatcher.java#L77

https://github.com/apache/pulsar/blob/master/pulsar-broker/src/main/java/org/apache/pulsar/broker/service/nonpersistent/NonPersistentDispatcherSingleActiveConsumer.java#L70

https://github.com/apache/pulsar/blob/master/pulsar-broker/src/main/java/org/apache/pulsar/broker/service/nonpersistent/NonPersistentDispatcherMultipleConsumers.java#L205

https://github.com/apache/pulsar/blob/master/pulsar-broker/src/main/java/org/apache/pulsar/broker/service/nonpersistent/NonPersistentStickyKeyDispatcherMultipleConsumers.java#L92

So this PR reduce the char[] creation on jvm heap while call subscription.toString().

### Modifications

Change implementation of subscription.toString()

pulsar-broker/src/main/java/org/apache/pulsar/broker/service/nonpersistent/NonPersistentSubscription.java

@@ -18,14 +18,13 @@
  */
 package org.apache.pulsar.broker.service.nonpersistent;
 
-import com.google.common.base.MoreObjects;
-
 import java.util.Collections;
 import java.util.List;
 import java.util.Map;
 import java.util.concurrent.CompletableFuture;
 import java.util.concurrent.atomic.AtomicIntegerFieldUpdater;
 
+import com.google.common.base.MoreObjects;
 import org.apache.bookkeeper.mledger.Entry;
 import org.apache.bookkeeper.mledger.Position;
 import org.apache.bookkeeper.mledger.impl.PositionImpl;
@@ -50,6 +49,7 @@ public class NonPersistentSubscription implements Subscription {
     private volatile NonPersistentDispatcher dispatcher;
     private final String topicName;
     private final String subName;
+    private final String fullName;
 
     private static final int FALSE = 0;
     private static final int TRUE = 1;
@@ -62,6 +62,7 @@ public NonPersistentSubscription(NonPersistentTopic topic, String subscriptionNa
         this.topic = topic;
         this.topicName = topic.getName();
         this.subName = subscriptionName;
+        this.fullName = MoreObjects.toStringHelper(this).add("topic", topicName).add("name", subName).toString();
         IS_FENCED_UPDATER.set(this, FALSE);
     }
 
@@ -155,7 +156,7 @@ public void acknowledgeMessage(List<Position> position, AckType ackType, Map<Str
 
     @Override
     public String toString() {
-        return MoreObjects.toStringHelper(this).add("topic", topicName).add("name", subName).toString();
+        return fullName;
     }
 
     @Override

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

@@ -20,8 +20,6 @@
 
 import static com.google.common.base.Preconditions.checkArgument;
 
-import com.google.common.base.MoreObjects;
-
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
@@ -33,6 +31,7 @@
 import java.util.concurrent.atomic.AtomicReferenceFieldUpdater;
 import java.util.stream.Collectors;
 
+import com.google.common.base.MoreObjects;
 import org.apache.bookkeeper.mledger.AsyncCallbacks;
 import org.apache.bookkeeper.mledger.AsyncCallbacks.ClearBacklogCallback;
 import org.apache.bookkeeper.mledger.AsyncCallbacks.DeleteCallback;
@@ -77,6 +76,7 @@ public class PersistentSubscription implements Subscription {
     protected volatile Dispatcher dispatcher;
     protected final String topicName;
     protected final String subName;
+    protected final String fullName;
 
     private static final int FALSE = 0;
     private static final int TRUE = 1;
@@ -139,6 +139,7 @@ public PersistentSubscription(PersistentTopic topic, String subscriptionName, Ma
         this.cursor = cursor;
         this.topicName = topic.getName();
         this.subName = subscriptionName;
+        this.fullName = MoreObjects.toStringHelper(this).add("topic", topicName).add("name", subName).toString();;
         this.expiryMonitor = new PersistentMessageExpiryMonitor(topicName, subscriptionName, cursor);
         this.setReplicated(replicated);
         IS_FENCED_UPDATER.set(this, FALSE);
@@ -470,7 +471,7 @@ public void deleteFailed(ManagedLedgerException exception, Object ctx) {
 
     @Override
     public String toString() {
-        return MoreObjects.toStringHelper(this).add("topic", topicName).add("name", subName).toString();
+        return fullName;
     }
 
     @Override