Handle RejectedExecution gracefully in TransportService

Today we might run into a rejected execution exception when
we shutdown the node while handling a transport exception. The
exception is run in a seperate thread but that thread might
not be able to execute due to the shutdown. Today we barf and fill
the logs with large exception. This commit catches this exception
and logs it as debug logging instead.

core/src/main/java/org/elasticsearch/transport/TransportService.java

@@ -332,9 +332,18 @@ public class TransportService extends AbstractLifecycleComponent<TransportServic
                 // callback that an exception happened, but on a different thread since we don't
                 // want handlers to worry about stack overflows
                 final SendRequestTransportException sendRequestException = new SendRequestTransportException(node, action, e);
-                threadPool.executor(ThreadPool.Names.GENERIC).execute(new Runnable() {
+                threadPool.executor(ThreadPool.Names.GENERIC).execute(new AbstractRunnable() {
                     @Override
-                    public void run() {
+                    public void onRejection(Throwable t) {
+                        // if we get rejected during node shutdown we don't wanna bubble it up
+                        logger.debug("failed to notify response handler on rejection", t);
+                    }
+                    @Override
+                    public void onFailure(Throwable t) {
+                        logger.warn("failed to notify response handler on exception", t);
+                    }
+                    @Override
+                    protected void doRun() throws Exception {
                         holderToNotify.handler().handleException(sendRequestException);
                     }
                 });