JIRA: https://jira.spring.io/browse/INT-4038
General format for node name: bean.method#n
The #n is needed to disambiguate methods with the same name.
Polishing according PR comments
* Fix generic type for `Collections.unmodifiableMap` usage
* Change the gateway method logic in the `IntegrationGraphServer` to include the method signature in the node name to distinguish them as unique
* Move `MutableMessageBuilderFactoryTests` into the separate nested `mutable` package since `@IntegrationComponentScan` sees a new `@MessagingGateway` in the `IntegrationGraphServerTests`.
See https://jira.spring.io/browse/INT-4040
* Document the `@MessagingGateway` representation in the `graph.adoc`
43b83bec53