INT-4196: MongoOutGateway: add CollectionCallback

JIRA: https://jira.spring.io/browse/INT-4196

Add `CollectionCallback` option to the `MongoDbOutboundGateway`

Add XML support to CollectionCallback

Fix PR comments

Fix issue with javadoc parsing

* Polishing code style, JavaDocs and some Docs

src/reference/asciidoc/mongodb.adoc

@@ -318,6 +318,8 @@ The Message payload and headers can be used to specify the query, as well as col
 If this attribute is not provided the default value is `org.bson.Document`;
 * `query` or `query-expression` - specifies the MongoDb query.
 Please refer to http://www.mongodb.org/display/DOCS/Querying[MongoDB documentation] for more query samples.
+* `collection-callback` - reference to an instance of `org.springframework.data.mongodb.core.CollectionCallback`
+(NOTE: you can not have both collection-callback and any of the query attributes).
 
 ==== Configuring with Java Configuration
 
@@ -337,16 +339,6 @@ public class MongoDbJavaApplication {
     @Autowired
     private MongoDbFactory mongoDbFactory;
 
-    @Bean
-    public MessageChannel requestChannel() {
-        return new DirectChannel();
-    }
-
-    @Bean
-    public MessageChannel replyChannel() {
-        return new QueueChannel(5);
-    }
-
     @Bean
     @ServiceActivator(inputChannel = "requestChannel")
     public MessageHandler mongoDbOutboundGateway() {
@@ -356,7 +348,6 @@ public class MongoDbJavaApplication {
         gateway.setExpectSingleResult(true);
         gateway.setEntityClass(Person.class);
         gateway.setOutputChannelName("replyChannel");
-
         return gateway;
     }
 
@@ -406,4 +397,17 @@ public class MongoDbJavaApplication {
     }
 
 }
+----
+
+Alternatively to the `query` and `query-expression` properties, you can specify other database operations through
+the `collectionCallback` property.
+The following example specifies a count operation:
+
+[source, java]
+----
+private MongoDbOutboundGatewaySpec collectionCallbackOutboundGateway() {
+    return MongoDb.outboundGateway(this.mongoDbFactory, this.mongoConverter)
+            .collectionCallback(MongoCollection::count)
+            .collectionName("foo");
+    }
 ----