fix: Fix possible exception-related ByteBuf leak (#99)

#### Motivation

Some ByteBuf leak errors were reported in a production deployment of model-mesh as described in #97.

#### Modifications

In `ModelMeshApi`, ensure that the response is released if a runtime exception is thrown prior to it being sent successfully.

#### Result

Hopefully leak closed

Signed-off-by: Nick Hill <nickhill@us.ibm.com>

src/main/java/com/ibm/watson/modelmesh/ModelMeshApi.java

@@ -768,14 +768,17 @@ public void onHalfClose() {
                         evictMethodDescriptor(methodName);
                     }
                 } finally {
+                    final boolean releaseResponse = status != OK;
                     if (payloadProcessor != null) {
                         ByteBuf data = null;
                         Metadata metadata = null;
                         if (response != null) {
                             data = response.data.readerIndex(respReaderIndex);
                             metadata = response.metadata;
                         }
-                        processPayload(data, requestId, modelId, methodName, metadata, status, false);
+                        processPayload(data, requestId, modelId, methodName, metadata, status, releaseResponse);
+                    } else if (releaseResponse && response != null) {
+                        response.release();
                     }
                     ReleaseAfterResponse.releaseAll();
                     clearThreadLocals();
@@ -805,7 +808,7 @@ private void processPayload(ByteBuf data, String payloadId, String modelId, Stri
                 try {
                     assert payloadProcessor != null;
                     if (!takeOwnership) {
-                        data.retain();
+                        ReferenceCountUtil.retain(data);
                     }
                     payload = new Payload(payloadId, modelId, methodName, metadata, data, status);
                     if (payloadProcessor.process(payload)) {