JIRA: https://jira.spring.io/browse/INT-4208
Since `AsyncRestTemplate` is deprecated in Spring 5.0, it doesn't make
sense to promote that feature via our new `AsyncHttpRequestExecutingMessageHandler`
component
* Rework (and rename) `AsyncHttpRequestExecutingMessageHandler` to
`ReactiveHttpRequestExecutingMessageHandler` and make it based on the
WebFlux `WebClient`
* Fix Java DSL (`ReactiveHttpMessageHandlerSpec`) and all tests according a new
logic in the `ReactiveHttpRequestExecutingMessageHandler`
* Fix XML namespace support to use new `ReactiveHttpRequestExecutingMessageHandler`
and don't expose unused options like `converters` and `request-factory`
* Fix `What's New` and `http.adoc`
* To remain with `async` mode for the `ReactiveHttpRequestExecutingMessageHandler`
behavior support fix `AbstractMessageProducingHandler` to adapt reply `Mono`
to the `SettableListenableFuture`
* Introduce new `reactive` behavior for the `AbstractMessageProducingHandler`
when `outputChannel` is `ReactiveSubscribableChannel` and perform
`.subscribeTo(Publisher)` for `Publisher` reply
* Upgrade to Spring AMQP `2.0 M3`
* Downgrade to Spring Security `4.2.2`
Address PR comments
Add async error handling to the one-way case
Minor polishing and checkstyle fix
19c5402079