* Hide `toReactivePublisher()` from lambdas
When an `IntegrationFlow` is declared via lambda, it is impossible
to materialize it as a `Publisher` since we lose a Lambda context and
finish with a `NoSuchBeanException` when we would like to inject that
`Publisher`
* Make `IntegrationFlowDefinition.toReactivePublisher()` as `protected`
and expose it on the `IntegrationFlowBuilder` level
* Use `Channels` as a singleton: no reason in overhead for its instances
* Some code style polishing for `IntegrationFlowDefinition`
* Restore `Channels.super()` call
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