Bump Pulsar to 2.8.0.15 (#692)

### Motivation
apache/pulsar#11804 added the custom implementations of `equals` and `hashCode` methods of `ServiceCnx`, which made KoP's spotbugs check fail with 

> Medium: io.streamnative.pulsar.handlers.kop.InternalServerCnx doesn't override org.apache.pulsar.broker.service.ServerCnx.equals(Object) [io.streamnative.pulsar.handlers.kop.InternalServerCnx] At InternalServerCnx.java:[line 1] EQ_DOESNT_OVERRIDE_EQUALS

### Modifications
This PR adds the overrided `equals` and `hashCode` methods by calling base class' same name methods simply. On the other hand, since the new `equals` and `hashCode` methods are based on `ctx` field and they are called when removing `Producer` from `Topic`, this PR also updates the `ctx` field of `InternalServerCnx`.

kafka-impl/src/main/java/io/streamnative/pulsar/handlers/kop/InternalServerCnx.java

@@ -58,6 +58,7 @@ public void closeProducer(Producer producer) {
     // called after channel active
     public void updateCtx() {
         this.remoteAddress = kafkaRequestHandler.remoteAddress;
+        this.ctx = kafkaRequestHandler.ctx;
     }
 
     @Override
@@ -74,4 +75,14 @@ public void disableCnxAutoRead() {
     public void cancelPublishBufferLimiting() {
         // do nothing is this mock
     }
+
+    @Override
+    public boolean equals(Object o) {
+        return super.equals(o);
+    }
+
+    @Override
+    public int hashCode() {
+        return super.hashCode();
+    }
 }

pom.xml

@@ -46,7 +46,7 @@
     <lombok.version>1.18.4</lombok.version>
     <mockito.version>2.22.0</mockito.version>
     <pulsar.group.id>io.streamnative</pulsar.group.id>
-    <pulsar.version>2.8.0.3</pulsar.version>
+    <pulsar.version>2.8.0.15</pulsar.version>
     <slf4j.version>1.7.25</slf4j.version>
     <spotbugs-annotations.version>3.1.8</spotbugs-annotations.version>
     <testcontainers.version>1.15.1</testcontainers.version>