INT-3555: Fix Header Channel Mapping

JIRA: https://jira.spring.io/browse/INT-3555

Previously, `replyChannel` and `errorChannel` headers were
unconditionally dropped in `AbstractHeaderMapper`. This was
changed in 4.1.0.M1 to allow these headers to be transmitted
in the case that they were translated to a String by a
`HeaderChannelRegistry`.

However, we should still drop these headers if they reference live
`MessageChannel` instances.
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