INT-3521 completions for ChannelInterceptorList

JIRA: https://jira.spring.io/browse/INT-3521

INT-3521: Rework invocation `index` to the `Deque` of invoked interceptors

Conflicts:
	src/reference/docbook/whats-new.xml

INT-3521: Address PR comments

Create an `interceptorStack` only if there are `interceptor` on the channel.
Invoke `afterSend(Receive)Completion` only `if (interceptorStack != null)`

Minor Doc Polishing

src/reference/docbook/channel.xml

@@ -306,9 +306,13 @@
 
     void postSend(Message<?> message, MessageChannel channel, boolean sent);
 
+    void afterSendCompletion(Message<?> message, MessageChannel channel, boolean sent, Exception ex);
+
     boolean preReceive(MessageChannel channel);
 
     Message<?> postReceive(Message<?> message, MessageChannel channel);
+
+    void afterReceiveCompletion(Message<?> message, MessageChannel channel, Exception ex);
 }]]></programlisting>
       After implementing the interface, registering the interceptor with a channel is just a matter of calling:
       <programlisting language="java">channel.addInterceptor(someChannelInterceptor);</programlisting>
@@ -363,6 +367,14 @@
         precede postReceive.
       </tip>
     </para>
+    <para>
+        Starting with <emphasis>Spring Framework 4.1</emphasis> and Spring Integration 4.1, the
+        <interfacename>ChannelInterceptor</interfacename> provides new methods - <code>afterSendCompletion()</code>
+        and <code>afterReceiveCompletion()</code>. They are invoked after <code>send()/receive()</code> calls,
+        regardless of any exception that is raised, thus allowing for resource cleanup. Note, the
+        Channel invokes these methods on the ChannelInterceptor List in the reverse order of the
+        initial <code>preSend()/preReceive()</code> calls.
+    </para>
   </section>
 
   <section id="channel-template">

src/reference/docbook/whats-new.xml

@@ -272,5 +272,14 @@
 				See <xref linkend="channel-configuration-queuechannel"/>.
 			</para>
 		</section>
+		<section id="4.1-channel-interceptor">
+			<title>ChannelInterceptor Changes</title>
+			<para>
+				The <interfacename>ChannelInterceptor</interfacename> now supports additional
+				<code>afterSendCompletion()</code> and <code>afterReceiveCompletion()</code> methods.
+				See <xref linkend="channel-interceptors"/>.
+			</para>
+		</section>
+
 	</section>
 </chapter>