JIRA: https://jira.spring.io/browse/INT-4300
* Add `ReactiveHttpInboundEndpoint` based on the WebFlux foundation
* Extract `BaseHttpInboundEndpoint` for common
HTTP Inbound Channel Adapters options
* Make `spring-webmvc` and `spring-webflux` as `optional` dependencies
to let end-user to choose
* Refactor `HttpContextUtils` to include constants
for newly added WebFlux support
* Introduce `BaseHttpInboundEndpoint.setRequestPayloadTypeClass()`
for raw `Class<?>` and modify existing `setRequestPayloadType()`
for the `ResolvableType`
* Refactor existing MVC tests and XML components parsers to use
new `setRequestPayloadTypeClass()`
* Add `MessagingGatewaySupport.sendAndReceiveMessageReactive()` to get
a reply from downstream flow reactive back-pressure manner
* Add `IntegrationHandlerResultHandler` implementation to let WebFlux
infrastructure to handle the `Mono<Void>` from the `ReactiveHttpInboundEndpoint`
properly
* Fix `JdbcLockRegistryLeaderInitiatorTests` race condition to assert
the `initiator1` is elected eventually after yielding when the `initiator2`
is stopped
* Fix JavaDocs issue in the `HttpRequestHandlingMessagingGateway`
* Move all the "hard" logic in the `MessagingGatewaySupport#doSendAndReceiveMessageReactive`
to the `Mono` chain ensuring back-pressure when `sendAndReceiveMessageReactive()` is
called not from the Reactive Stream
Add test-case to demonstrate SSE
JIRA: https://jira.spring.io/browse/INT-3625
Some polishing and optimization for the
`MessagingGatewaySupport.doSendAndReceiveMessageReactive()`
More optimization for `MessagingGatewaySupport`
* Upgrade to Reactor 3.1 M3
* Document WebFlux-based components
Minor Doc Polishing
d4a99919ed