* Infrastructure for ReactiveMessageHandler
We have now a `ReactiveMongoDbStoringMessageHandler` which implements
a `ReactiveMessageHandler`, but not a `MessageHandler` for possible
deferred subscriptions to the returned Reactor type
We don't have a proper application context processing for this
new type of message handlers
* Change a `ConsumerEndpointFactoryBean` to apply an `MH` and `RMH`
as possible types for handler
* Introduce a `ReactiveMessageHandlerAdapter` to wrap an `RMH`
into a `MH` for synchronous calls in the regular consumer endpoints
* Wrap an `RMH` into a `ReactiveMessageHandlerAdapter` for regular
endpoints and unwrap for `ReactiveStreamsConsumer`
* Add `RMH`-based ctor into `ReactiveStreamsConsumer` for target
reactive streams composition (`flatMap()` on the `RMH`)
* Remove a `DelegatingSubscriber` from the `ReactiveStreamsConsumer`
in favor of direct calls from the `doOnSubscribe()`, `doOnComplete()`
& `doOnNext()`
* Add an `onErrorContinue()` to handle per-message errors, but don't
cancel the whole source `Publisher`
* Use `Disposable` from the `subscribe()` to cancel in the `stop()`
- recommended way in Reactor
* Use `onErrorContinue()` in the `FluxMessageChannel` instead of
`try..catch` in the `doOnNext()` - for possible `onErrorStop()`
in the provided upstream `Publisher`
* Handle `RMH` in the `ServiceActivatorFactoryBean` as a direct handler
as well with wrapping into `ReactiveMessageHandlerAdapter` for return.
The `ConsumerEndpointFactoryBean` extracts an `RMH` from the adapter
for the `ReactiveStreamsConsumer` anyway
* Add XML parsing test for `ReactiveMongoDbStoringMessageHandler`
* Add `log4j-slf4j-impl` for all the test runtime since `slf4j-api`
comes as a transitive dependency from many places
* * Fix conflicts after rebasing to master
* * Fix typo in warn message
* Change `Assert.state()` to `Assert.isTrue()`
for `ConsumerEndpointFactoryBean.setHandler()`
* * Fix `ConsumerEndpointFactoryBean` when reactive and no advice-chain
* Fix race condition in the
`ReactiveMongoDbStoringMessageHandlerTests.testReactiveMongoMessageHandlerFromApplicationContext()`
* * Handle `ReactiveMessageHandler` in Java DSL.
Essentially request a wrapping into `ReactiveMessageHandlerAdapter`.
Describe such a requirements in the `ReactiveMessageHandlerAdapter` JavaDocs
* Some Java DSL test polishing
* Add Java DSL for `ReactiveMongoDbStoringMessageHandler`
* Propagate missed `ApplicationContext` population into an internally
created `ReactiveMongoTemplate` in the `ReactiveMongoDbStoringMessageHandler`
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