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`.