INT-4129: Add Discard Channel to Barrier Handler

JIRA: https://jira.spring.io/browse/INT-4129

Discard late arriving triggers.

* Minor code style polishing

src/reference/asciidoc/barrier.adoc

@@ -44,6 +44,7 @@ An exception is thrown if a second thread arrives with the same correlation.
 <int:barrier id="barrier1" input-channel="in" output-channel="out"
         correlation-strategy-expression="headers['myHeader']"
         output-processor="myOutputProcessor"
+        discard-channel="lateTriggerChannel"
         timeout="10000">
 </int:barrier>
 
@@ -55,6 +56,7 @@ Either the thread sending a message to `in` or the one sending a message to `rel
 up to 10 seconds until the other arrives.
 When the message is released, the `out` channel will be sent a message combining the result of invoking the
 custom `MessageGroupProcessor` bean `myOutputProcessor`.
+If the main thread times out and a trigger arrives later, you can configure a discard channel to which the late trigger will be sent.
 Java configuration is shown below.
 
 [source, java]
@@ -68,6 +70,7 @@ public class Config {
     public BarrierMessageHandler barrier() {
         BarrierMessageHandler barrier = new BarrierMessageHandler(10000);
         barrier.setOutputChannel(out());
+        barrier.setDiscardChannel(lateTriggers());
         return barrier;
     }
 

src/reference/asciidoc/whats-new.adoc

@@ -50,3 +50,8 @@ See <<global-properties>> for more information.
 
 There is a new option on the `CharacterStreamReadingMessageSource` to allow it to be used to "pipe" stdin and publish an application event when the pipe is closed.
 See <<stream-reading>> for more information.
+
+==== Barrier Changes
+
+The `BarrierMessageHandler` now supports a discard channel to which late-arriving trigger messages are sent.
+See <<barrier>> for more information.