Refactor FilterWebHandler and DefaultWebFilterChain

The chain is initialized once and re-used vs creating the "next" chains
on every request.

spring-web/src/test/java/org/springframework/web/server/handler/FilteringWebHandlerTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2016 the original author or authors.
+ * Copyright 2002-2018 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -42,6 +42,7 @@ import static org.junit.Assert.assertNotNull;
 import static org.junit.Assert.assertTrue;
 
 /**
+ * Unit tests for {@link FilteringWebHandler}.
  * @author Rossen Stoyanchev
  */
 public class FilteringWebHandlerTests {