Fix handler name on message not fully read

Today when a message is not fully read on a response, we log (among
other details) the handler name. Unfortunately, if the handler is a
wrapper, all that we see is

o.e.x.s.t.TransportService$ContextRestoreResponseHandler@7446ba18

completely losing the offending handler. This commit adds an override
for TransportService$ContextRestoreResponseHandler#toString so that the
underlying offender can be discovered.

Relates elastic/elasticsearch#4043

Original commit: elastic/x-pack-elasticsearch@3eb1003d5b

elasticsearch/src/main/java/org/elasticsearch/xpack/security/transport/SecurityServerTransportInterceptor.java

@@ -259,6 +259,7 @@ public class SecurityServerTransportInterceptor implements TransportInterceptor
      * are invoked we restore the context.
      */
     static final class ContextRestoreResponseHandler<T extends TransportResponse> implements TransportResponseHandler<T> {
+
         private final TransportResponseHandler<T> delegate;
         private final ThreadContext.StoredContext context;
         private final ThreadContext threadContext;
@@ -296,5 +297,12 @@ public class SecurityServerTransportInterceptor implements TransportInterceptor
         public String executor() {
             return delegate.executor();
         }
+
+        @Override
+        public String toString() {
+            return getClass().getName() + "/" + delegate.toString();
+        }
+
     }
+
 }