Provide the Future#flatMap method as alias of Future#compose method - closes #3145

src/main/java/io/vertx/core/Future.java

@@ -150,6 +150,13 @@ public interface Future<T> extends AsyncResult<T> {
   @Override
   boolean failed();
 
+  /**
+   * Alias for {@link #compose(Function)}.
+   */
+  default <U> Future<U> flatMap(Function<T, Future<U>> mapper) {
+    return compose(mapper);
+  }
+
   /**
    * Compose this future with a {@code mapper} function.<p>
    *