INT-3944: Async JMS Outbound Gateway

JIRA: https://jira.spring.io/browse/INT-3944

Polishing (PR comments) and Doc Polish

Polishing - PR Comments

Remove custom `async` boolean in favor of the now public setter.

Add exception to `JmsException` hierarchy.

INT-3944: Polishing

* Rename `asyncReplySupported` just to `async`. As well as its getter and setter. Plus fix docs on the matter
* Polishing for the `JmsOutboundGateway` according PR comments
* Rework `JmsOutboundGateway` to return `AbstractIntegrationMessageBuilder` instead of `Message`
since `AbstractMessageProducingHandler` rebuilds `Message` for a new one to copy headers from request
* Add `getPayload()` and `getHeaders()` to the `AbstractIntegrationMessageBuilder` to make the `Routing Slip`
users happy, when the `AbstractIntegrationMessageBuilder` is pushed to the `Routing Slip path` function
* Fix race condition in the `ChatMessageListeningEndpointTests`: the `stanza` parsing is done in the different Thread.

Doc Polishing (Routing Slip)

Fix timing issue in the `LastModifiedFileListFilterTests`:
`Thread.sleep()` not always reflects the reality.
Switch to the "past simulation" via explicit `File.setLastModified()`

spring-integration-file/src/test/java/org/springframework/integration/file/filters/LastModifiedFileListFilterTests.java

@@ -45,7 +45,7 @@ public class LastModifiedFileListFilterTests {
 		fileOutputStream.write("x".getBytes());
 		fileOutputStream.close();
 		assertEquals(0, filter.filterFiles(new File[] { foo }).size());
-		Thread.sleep(2000);
+		foo.setLastModified(System.currentTimeMillis() - 10000);
 		assertEquals(1, filter.filterFiles(new File[] { foo }).size());
 	}