INT-3485: Remove final from `AE.stop(Runnable)`

JIRA: https://jira.spring.io/browse/INT-3485

Delegate to doStop() instead of overriding stop() directly

Delegated call to doStop() rather than stop(), added author tag and added unit test for custom doStop(runnable)

Polishing
INT-3479 Improve ServiceActivatingHandler.toString

JIRA: https://jira.spring.io/browse/INT-3479

- Remove null `beanName` in `AbstractStandardMessageHandlerFactoryBean`
- Change `ServiceActivatingHandler.toString()`
- Fix aggregator test timing
INT-3478 SftpPAOnceFileListFilter use getFilename

JIRA: https://jira.spring.io/browse/INT-3478

INT-3478: add test case

Polishing
INT-3477: Add Reactor Promise<?> for Gateway

JIRA: https://jira.spring.io/browse/INT-3477

Conflicts:
	spring-integration-core/src/test/java/org/springframework/integration/configuration/EnableIntegrationTests.java
INT-3477: Fix `reactorEnvironment` propagation

* `MessagingGatewayRegistrar` parser the value for the `reactorEnvironment`
* Provide more interest test-case
* Polishing docs
INT-3477 Require `Environment` in case of Promise

* Do not use Reactor `Environment` as default instance.
* Require `Environment` reference, when is `Promise` method
* Polishing for tests
* Apply Gary's polishing for docs
INT-3477: Assert.notNull -> Assert.state
INT-3477: PR review