Polish Servlet async request processing

* Clarify semantics and behavior of AsyncWebRequest methods in most cases
making a best effort and not raising an exception if async processing
has completed for example due to a timeout. The startAsync() method is
still protected with various checks and will raise ISE under a number
of conditions.
* Return 503 (service unavailable) when requests time out.
* Logging improvements.

Issue: SPR-8517

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/RequestMappingHandlerAdapter.java

@@ -398,10 +398,10 @@ public class RequestMappingHandlerAdapter extends AbstractHandlerMethodAdapter i
 	}
 
 	/**
-	 * Set the timeout for asynchronous request processing. When the timeout
-	 * begins depends on the underlying async technology. With the Servlet 3
-	 * async support the timeout begins after the main processing thread has
-	 * exited and has been returned to the container pool.
+	 * Set the timeout for asynchronous request processing in milliseconds.
+	 * When the timeout begins depends on the underlying async technology.
+	 * With the Servlet 3 async support the timeout begins after the main
+	 * processing thread has exited and has been returned to the container pool.
 	 * <p>If a value is not provided, the default timeout of the underlying
 	 * async technology is used (10 seconds on Tomcat with Servlet 3 async).
 	 */

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/ServletInvocableHandlerMethod.java

@@ -126,6 +126,7 @@ public class ServletInvocableHandlerMethod extends InvocableHandlerMethod {
 
 		return new AbstractDelegatingCallable() {
 			public Object call() throws Exception {
+				mavContainer.setRequestHandled(false);
 				new CallableHandlerMethod(getNextCallable()).invokeAndHandle(webRequest, mavContainer, providedArgs);
 				return null;
 			}