INT-3554: Add `Expression` Setters

JIRA: https://jira.spring.io/browse/INT-3554

**Cherry-pick to 4.0.x**

Polishing
INT-3550: RoutingSlip Improvements

JIRA: https://jira.spring.io/browse/INT-3550

* Make `RoutingSlipRouteStrategy#getNextPath` as `Object` return type. to allow to produce `MessageChannel` result, not only `beanName`
* Make `RoutingSlipHeaderValueMessageProcessor` ctor to accept `Object... routingSlipPath` instead of just String.
It is useful from JavaConfig, when we can use `RoutingSlipRouteStrategy` `@Bean` reference.
* Add JavaConfig test case to demonstrate how `RoutingSlipRouteStrategy` can get deal with inline `FixedSubscriberChannel`
and Lambdas together with Reactor Streams.

INT-3550: Polishing according PR comments

* Fix `AbstractMessageProducingHandler` to check if `nextPath` isn't empty String
* Add `RoutingSlipHeaderValueMessageProcessor` ctor check for the `routingSlipPath` entries types
* Add `RoutingSlipRouteStrategy` JavaDocs regarding the loop of strategy invocation
* Add `Process Manager` doc

Doc Polishing.
INT-3554: Expression Setters Polishing

JIRA: https://jira.spring.io/browse/INT-3554
INT-3556: Payload Conversion Documentation

JIRA: https://jira.spring.io/browse/INT-3556

Clarify when a `GenericConverter` might be needed rather
than a simple `Converter`.
INT-3551: Idempotent Receiver: Add value-strategy

JIRA: https://jira.spring.io/browse/INT-3551

* Rename `MetadataKeyStrategy` -> `MetadataEntryStrategy`
* Add `valueStrategy` to the `MetadataStoreSelector`
* Add `value-strategy` and `value-expression` to the `<idempotent-receiver>`

INT-3551: Add `@IR` support on service methods

Rework `MetadataKeyStrategy` just to the `MessageProcessor`
Fix Docs

Minor Doc Polishing.
INT-3557 Clarify MessageHeaders Constants

JIRA: https://jira.spring.io/browse/INT-3557

Add class names and code tags.

Add missing header constants.

Polishing for `timestamp` header doc.

Fix JavaDocs issues: https://build.spring.io/browse/INT-EIGHTAPI-296
Fix Docs issue

https://build.spring.io/browse/INT-B41-152
INT-3531 Update to Spring Framework 4.1.2

JIRA: https://jira.spring.io/browse/INT-3531

INT-3531 Fix Stub AMQP Channel

Compatibility with 3.4.x amqp-client.
[artifactory-release] Release version 4.1.0.RELEASE
[artifactory-release] Next development version
ChannelAdapterParserTests - Increase Receive Times

Fix occasional CI build plan failures.
Add BF Required to Test Builds

Previously, the environment variable that ensures all message creators have
a `BeanFactory` required setting the `SI_FATAL_WHEN_NO_BEANFACTORY` environment
variable to `true`.

Add this variable to `build.gradle`, enforcing the test for all builds.
INT-3565 ClassCastException with Proxy and DEBUG

JIRA: https://jira.spring.io/browse/INT-3565

Cast to `IntegrationObjectSupport` fails if the handler is
already proxied in `AbstractSimpleMessageHandlerFactoryBean`.

INT-3565 Polishing

- Fix test.
- enhance test to show that directly bound message handler beans are not advised if already proxied.