INT-3538: Docs for Scatter-Gather

JIRA: https://jira.spring.io/browse/INT-3538

Phase #1: `Scatter-Gather`

S-G Doc Polishing

INT-3538 Add `requires-reply` to `scatter-gather`

Final S-G Doc Polish
INT-3538: Docs for Routing Slip

JIRA: https://jira.spring.io/browse/INT-3538

INT-3538-2: `RoutingSlipHeaderValueMessageProcessor` improvement

Since `HeaderEnricher` populates `beanFactory` to its `HeaderValueMessageProcessor`s,
there is no reason to specify `RoutingSlipHeaderValueMessageProcessor` as a separate bean.
Therefore the `IntegrationEvaluationContextAware` may not be reached.
Remove its support from `RoutingSlipHeaderValueMessageProcessor` and just rely on the `setBeanFactory`

RS Doc Polishing
INT-3538-3: Docs for `Idempotent Receiver`

JIRA: https://jira.spring.io/browse/INT-3538

I-R Doc Polishing
INT-3545: Don't AutoStart HeaderChannelRegistry

JIRA: https://jira.spring.io/browse/INT-3545

Previously, the `DefaultHeaderChannelRegistry` was auto-started.
This caused the reaper to run and eventually start all the
`taskScheduler` threads, even if the registry was not being used.

- Defer the `start()` until the first channel is stored.
- Do not start the reaper if the bean has been explicitly stopped.
- Deprecate the implementation of `SmartLifecycle`.