* Add `ConsumerSubscriber` to adapt `Consumer<?>` to the `Subscriber<?>`
* Add `SubscribableChannelPublisherAdapter` to adapt `SubscribableChannel` to `Publisher<?>` via `Flux.create()` on subscription
* Add `PollableChannelPublisherAdapter` to adapt `PollableChannel` to `Publisher<?>` via `Mono.delayMillis()` and `flatMap()` for `channel.receive()`
Use `concatMap()` for `PollableChannel`
Iterate `PollableChannel` until there is a data
Fix Lambda signature error via explicit class declaration:
```
java.lang.ClassFormatError: Duplicate field name&signature in class file org/springframework/integration/endpoint/ReactiveConsumer$PollableChannelPublisherAdapter$1
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(ClassLoader.java:760)
at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
at java.net.URLClassLoader.defineClass(URLClassLoader.java:467)
at java.net.URLClassLoader.access$100(URLClassLoader.java:73)
at java.net.URLClassLoader$1.run(URLClassLoader.java:368)
at java.net.URLClassLoader$1.run(URLClassLoader.java:362)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:361)
at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:331)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
at org.springframework.integration.endpoint.ReactiveConsumer$PollableChannelPublisherAdapter.lambda$new$2(ReactiveConsumer.java:163)
```
Looks like Java bug:
Move `Iterator<Message<?>>` instance to the `subscribe()` to avoid race conditions when we have several subscribers.
In other words make `Iterator<Message<?>>` subscriber-specific, but at the same time avoid re-instantiation for each `Flux.concatMap()` call caused by the previous `repeat()`
Upgrade to Reactor 3.0.3 and others
* Make fixes according Reactor 3.0.3 changes
* Remove redundant `TestSubscriber` in favor of `StepVerifier` and `mock(Subscriber)`
* Rework `PublisherIntegrationFlow` Reactive Streams implementation to the out-of-the-box `ReactiveConsumer` and `ReactiveChannel`
ba576bbc0d