Ensure broker is fully boostrapped before load manager register itself (

apache#2935)

### Motivation

In some cases the broker can immediately gets assigned traffic before it's fully boostrapped. 

This happens because the load manager is registering the broker in ZK before some of the initialization steps are completed. 

This results in NPE, like : 

```
Caused by: java.lang.NullPointerException
	at org.apache.pulsar.broker.service.persistent.PersistentTopic.hasSchema(PersistentTopic.java:1815) ~[org.apache.pulsar-pulsar-broker-2.2.0-streamlio-22.jar:2.2.0-streamlio-22]
	at org.apache.pulsar.broker.service.ServerCnx.lambda$25(ServerCnx.java:836) ~[org.apache.pulsar-pulsar-broker-2.2.0-streamlio-22.jar:2.2.0-streamlio-22]
	at java.util.concurrent.CompletableFuture.uniAccept(CompletableFuture.java:656) ~[?:1.8.0_181]
```

### Modifications

 * Register the broker in ZK only after the full start sequence has been done. This will ensure other brokers will not discover this broker before it's ready.
 * Expose the "is ready" state in the VipStatus -- This will be used to make sure the load balancer will not direct any lookup request to the broker before it's ready.

pulsar-broker-common/src/main/java/org/apache/pulsar/common/configuration/VipStatus.java

@@ -19,6 +19,7 @@
 package org.apache.pulsar.common.configuration;
 
 import java.io.File;
+import java.util.function.Supplier;
 
 import javax.servlet.ServletContext;
 import javax.ws.rs.GET;
@@ -34,18 +35,23 @@
 public class VipStatus {
 
     public static final String ATTRIBUTE_STATUS_FILE_PATH = "statusFilePath";
+    public static final String ATTRIBUTE_IS_READY_PROBE = "isReadyProbe";
 
     @Context
     protected ServletContext servletContext;
 
     @GET
     @Context
     public String checkStatus() {
-
         String statusFilePath = (String) servletContext.getAttribute(ATTRIBUTE_STATUS_FILE_PATH);
+        @SuppressWarnings("unchecked")
+        Supplier<Boolean> isReadyProbe = (Supplier<Boolean>) servletContext.getAttribute(ATTRIBUTE_IS_READY_PROBE);
+
+        boolean isReady = isReadyProbe != null ? isReadyProbe.get() : true;
+
         if (statusFilePath != null) {
             File statusFile = new File(statusFilePath);
-            if (statusFile.exists() && statusFile.isFile()) {
+            if (isReady && statusFile.exists() && statusFile.isFile()) {
                 return "OK";
             }
         }

pulsar-broker/src/main/java/org/apache/pulsar/broker/PulsarService.java

@@ -172,7 +172,7 @@ public enum State {
         Init, Started, Closed
     }
 
-    private State state;
+    private volatile State state;
 
     private final ReentrantLock mutex = new ReentrantLock();
     private final Condition isClosedCondition = mutex.newCondition();
@@ -355,8 +355,6 @@ public void start() throws PulsarServerException {
             // Start load management service (even if load balancing is disabled)
             this.loadManager.set(LoadManager.create(this));
 
-            this.startLoadManagementService();
-
             // needs load management service
             this.startNamespaceService();
 
@@ -369,6 +367,13 @@ public void start() throws PulsarServerException {
             attributeMap.put(WebService.ATTRIBUTE_PULSAR_NAME, this);
             Map<String, Object> vipAttributeMap = Maps.newHashMap();
             vipAttributeMap.put(VipStatus.ATTRIBUTE_STATUS_FILE_PATH, this.config.getStatusFilePath());
+            vipAttributeMap.put(VipStatus.ATTRIBUTE_IS_READY_PROBE, new Supplier<Boolean>() {
+                @Override
+                public Boolean get() {
+                    // Ensure the VIP status is only visible when the broker is fully initialized
+                    return state == State.Started;
+                }
+            });
             this.webService.addRestResources("/", VipStatus.class.getPackage().getName(), false, vipAttributeMap);
             this.webService.addRestResources("/", "org.apache.pulsar.broker.web", false, attributeMap);
             this.webService.addRestResources("/admin", "org.apache.pulsar.broker.admin.v1", true, attributeMap);
@@ -446,11 +451,16 @@ public synchronized void brokerIsAFollowerNow() {
 
             leaderElectionService.start();
 
+            schemaRegistryService = SchemaRegistryService.create(this);
+
             webService.start();
 
             this.metricsGenerator = new MetricsGenerator(this);
 
-            schemaRegistryService = SchemaRegistryService.create(this);
+            // By starting the Load manager service, the broker will also become visible
+            // to the rest of the broker by creating the registration z-node. This needs
+            // to be done only when the broker is fully operative.
+            this.startLoadManagementService();
 
             state = State.Started;
 
@@ -810,12 +820,12 @@ public synchronized PulsarAdmin getAdminClient() throws PulsarServerException {
                         .authentication( //
                                 conf.getBrokerClientAuthenticationPlugin(), //
                                 conf.getBrokerClientAuthenticationParameters());
-                
+
                 if (conf.isBrokerClientTlsEnabled()) {
                     builder.tlsTrustCertsFilePath(conf.getBrokerClientTrustCertsFilePath());
                     builder.allowTlsInsecureConnection(conf.isTlsAllowInsecureConnection());
                 }
-                
+
                 this.adminClient = builder.build();
                 LOG.info("Admin api url: " + adminApiUrl);
             } catch (Exception e) {