INT-4208: Support AsycRest in HTTP Module

https://jira.spring.io/browse/INT-4208
https://jira.spring.io/browse/INT-4076

Add `AsyncRestTemplate` support in `HttpRequestExecutingMessageHandler` and corresponding config/dsl support

* Polishing Docs

src/reference/asciidoc/http.adoc

@@ -5,7 +5,7 @@
 === Introduction
 
 The HTTP support allows for the execution of HTTP requests and the processing of inbound HTTP requests.
-Because interaction over HTTP is always synchronous, even if all that is returned is a 200 status code, the HTTP support consists of two gateway implementations: `HttpInboundEndpoint` and `HttpRequestExecutingMessageHandler`.
+the HTTP support consists of the following gateway implementations: `HttpInboundEndpoint`, `HttpRequestExecutingMessageHandler` and `AsyncHttpRequestExecutingMessageHandler`
 
 [[http-inbound]]
 === Http Inbound Components
@@ -125,6 +125,7 @@ The key that is used for that map entry by default is 'reply', but this can be o
 
 [[http-outbound]]
 === Http Outbound Components
+==== HttpRequestExecutingMessageHandler
 
 To configure the `HttpRequestExecutingMessageHandler` write a bean definition like this:
 
@@ -164,6 +165,7 @@ When set to true (default is false), a _Set-Cookie_ header received from the ser
 This header will then be used on subsequent sends.
 This enables simple stateful interactions, such as...
 
+
 `...->logonGateway->...->doWorkGateway->...->logoffGateway->...`
 
 If _transfer-cookies_ is false, any _Set-Cookie_ header received will remain as _Set-Cookie_ in the reply message, and will be dropped on subsequent sends.
@@ -190,6 +192,37 @@ The `expected-response-type` must be compatible with the (configured or default)
 Of course, this can be an abstract class, or even an interface (such as `java.io.Serializable` when using java serialization and `Content-Type: application/x-java-serialized-object`).
 =====
 
+==== AsyncHttpRequestExecutingMessageHandler
+
+The `AsyncHttpRequestExecutingMessageHandler` implementation is very similar to `HttpRequestExecutingMessageHandler` instead of delegating to a `AsyncRestTemplate`.
+To configure it, write a bean like this:
+
+[source,xml]
+----
+<bean id="httpAsyncOutbound"
+    class="org.springframework.integration.http.outbound.AsyncHttpRequestExecutingMessageHandler">
+     <constructor-arg value="http://localhost:8080/example" />
+     <property name="outputChannel" ref="responseChannel" />
+</bean>
+----
+
+You can configure the `AsyncClientHttpRequestFactory` instance to use:
+
+[source,xml]
+----
+<bean id="httpOutbound"
+    class="org.springframework.integration.http.outbound.AsyncHttpRequestExecutingMessageHandler">
+     <constructor-arg value="http://localhost:8080/example" />
+     <property name="outputChannel" ref="responseChannel" />
+     <property name="asyncRequestFactory" ref="customRequestFactory" />
+</bean>
+----
+
+By default the HTTP request will be generated using an instance of `SimpleClientHttpRequestFactory`.
+Use of the Apache Commons HTTP Async Client is also supported through the provided `HttpComponentsAsyncClientHttpRequestFactory` which can be injected as shown above.
+
+For other settings like cookie, converters, etc, please see <<HttpRequestExecutingMessageHandler>> above.
+
 [[http-namespace]]
 === HTTP Namespace Support
 
@@ -512,6 +545,34 @@ The configuration looks very similar to the gateway:
     auto-startup="false"/>
 ----
 
+If you want to execute the http request in an asynchronous way, you can use the `outbound-async-gateway` or `outbound-async-channel-adapter`.
+
+[source,xml]
+----
+<int-http:outbound-async-gateway id="asyncExample1"
+    request-channel="requests"
+    url="http://localhost/test"
+    http-method-expression="headers.httpMethod"
+    extract-request-payload="false"
+    expected-response-type-expression="payload"
+    charset="UTF-8"
+    async-request-factory="requestFactory"
+    reply-timeout="1234"
+    reply-channel="replies"/>
+    
+<int-http:outbound-async-channel-adapter id="asyncExample2"
+    url="http://localhost/example"
+    http-method="GET"
+    channel="requests"
+    charset="UTF-8"
+    extract-payload="false"
+    expected-response-type="java.lang.String"
+    async-request-factory="someRequestFactory"
+    order="3"
+    auto-startup="false"/>
+    
+----
+
 [NOTE]
 =====
 To specify the URL; you can use either the 'url' attribute or the 'url-expression' attribute.

src/reference/asciidoc/whats-new.adoc

@@ -14,6 +14,11 @@ development process.
 The new `MongoDbOutboundGateway` allows you to make queries to the database on demand by sending a message to its request channel.
 See <<mongodb-outbound-gateway>> for more information.
 
+==== HTTP Async Outbound Gateway and Channel Adapter
+
+The new `AsyncHttpRequestExecutingMessageHandler` adds support for `AsyncRestTemplate` for outbound channel adapter and gateway.
+See <<AsyncHttpRequestExecutingMessageHandler>> for more information.
+
 [[x5.0-general]]
 === General Changes