INT-4042: Support Known Router Channels in Graph

JIRA: https://jira.spring.io/browse/INT-4042

```
...
  }, {
    "nodeId" : 24,
    "name" : "integrationGraphServerTests.Config.router.router",
    "stats" : {
       ...
      }
    },
    "componentType" : "router",
    "output" : "discards",
    "input" : "four",
    "routes" : [ "barChannel", "bazChannel" ],
    "errors" : "myErrors"
  }, {
...
    "from" : 3,
    "to" : 24,
    "type" : "input"
  }, {
    "from" : 24,
    "to" : 7,
    "type" : "output"
  }, {
    "from" : 24,
    "to" : 2,
    "type" : "error"
  }, {
    "from" : 24,
    "to" : 5,
    "type" : "route"
  }, {
    "from" : 24,
    "to" : 6,
    "type" : "route"
...
```

Add Graph Support for RecipientListRouter

The RLR is not an AMMR; it's the only router outside of that class hierarchy.

Polishing

Add Dynamically Routed-to Channels to Graph

Add ExpressionBased and Expose Expression in Graph

Polishing

Add getExpressionString() to IOS

Add ExpressionCapable; Set Primary Expression

Polishing - PR Comments

Router/Expression Docs

* Simple Java Docs polishing

spring-integration-file/src/main/java/org/springframework/integration/file/remote/gateway/AbstractRemoteFileOutboundGateway.java

@@ -307,9 +307,11 @@ public abstract class AbstractRemoteFileOutboundGateway<F> extends AbstractReply
 		Assert.notNull(remoteFileTemplate, "'remoteFileTemplate' cannot be null");
 		this.remoteFileTemplate = remoteFileTemplate;
 		this.command = command;
+		Expression parsedExpression = new SpelExpressionParser().parseExpression(expression);
 		this.fileNameProcessor = new ExpressionEvaluatingMessageProcessor<String>(
-			new SpelExpressionParser().parseExpression(expression));
+			parsedExpression);
 		this.messageSessionCallback = null;
+		setPrimaryExpression(parsedExpression);
 	}
 
 	/**