The `FluxMessageChannel` can subscribe to any volatile `Publisher`.
For example, we can call Reactor Kafka `Sender.send()` for
input data and pass its result to the `FluxMessageChannel`
for on demand subscription.
These publishers are subscribed in the `FluxMessageChannel`
and their `Disposable` is stored in the internal `Disposable.Composite`
which currently only cleared on `destroy()`
* Extract `Disposable` from those internal `subscribe()` calls
into an `AtomicReference`.
* Use this `AtomicReference` in the `doOnTerminate()`
to remove from the `Disposable.Composite` and `dispose()`
when such a volatile `Publisher` is completed
**Cherry-pick to `6.0.x` & `5.5.x`**
# Conflicts:
# spring-integration-core/src/main/java/org/springframework/integration/channel/FluxMessageChannel.java
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