Empty stacktrace of TimeoutException (#202)

Empty stacktrace of `TimeoutException`

#### Problem

`TimeoutPublisher` creates a new `TimeoutException` instance for every timeout. This is costly when the system is under load and has a lot of timers that timeout, which is typical when handling latent outbound services.

#### Modification

Used a cached `TimeoutException` instance and empty the stacktrace of the exception.

#### Result

Less overhead of timeouts.

Author: NiteshKant

reactivesocket-publishers/src/main/java/io/reactivesocket/reactivestreams/extensions/internal/publishers/TimeoutPublisher.java

@@ -28,6 +28,16 @@
 
 public final class TimeoutPublisher<T> implements Px<T> {
 
+    @SuppressWarnings("ThrowableInstanceNeverThrown")
+    private static final TimeoutException timeoutException = new TimeoutException() {
+        private static final long serialVersionUID = 6195545973881750858L;
+
+        @Override
+        public synchronized Throwable fillInStackTrace() {
+            return this;
+        }
+    };
+
     private final Publisher<T> child;
     private final Scheduler scheduler;
     private final long timeout;
@@ -42,7 +52,7 @@ public TimeoutPublisher(Publisher<T> child, long timeout, TimeUnit unit, Schedul
 
     @Override
     public void subscribe(Subscriber<? super T> subscriber) {
-        Runnable onTimeout = () -> subscriber.onError(new TimeoutException());
+        Runnable onTimeout = () -> subscriber.onError(timeoutException);
         CancellableSubscriber<Void> timeoutSub = Subscribers.create(null, null, throwable -> onTimeout.run(),
                                                                     onTimeout, null);
         Runnable cancelTimeout = () -> timeoutSub.cancel();