* Use `EmitterProcessor` for Channels adaptation
Related https://github.com/spring-cloud/spring-cloud-stream/issues/1835
To honor a back-pressure after `MessageChannel` adaptation it is better
to use an `EmitterProcessor.create(1)` instead of `Flux.create()`.
This way whenever an emitter buffer is full, we block upstream producer
and don't allow it to produce more messages
**Cherry-pick to 5.1.x**
* * Wrap every new subscription into a `Flux.defer()`
* Fix `ReactiveStreamsConsumerTests` to use a new `Subscription` after
each `stop()/start()` on the `ReactiveStreamsConsumer`
* * Remove unused imports
36c9f72a9d