Add JavaDSL for ReactiveMongoDbMessageSource

* Implement `ReactiveMongoDbMessageSourceSpec` and factories for it
* Rework `ReactiveMongoDbMessageSourceTests` to use new Java DSL for `ReactiveMongoDbMessageSource`
* Document changes

src/reference/asciidoc/mongodb.adoc

@@ -544,3 +544,26 @@ public IntegrationFlow reactiveMongoDbFlow(ReactiveMongoDatabaseFactory mongoDbF
 
 In this sample we are going to connect to the MongoDb via provided `ReactiveMongoDatabaseFactory` and store a data from request message into a default collection with the `data` name.
 The real operation is going to be performed on-demand from the reactive stream composition in the internally created `ReactiveStreamsConsumer`.
+
+The `ReactiveMongoDbMessageSource` is an `AbstractMessageSource` implementation based on the provided `ReactiveMongoDatabaseFactory` or `ReactiveMongoOperations` and MongoDb query (or expression), calls `find()` or `findOne()` operation according an `expectSingleResult` option with an expected `entityClass` type to convert a query result.
+A query execution and result evaluation is performed on demand when `Publisher` (`Flux` or `Mono` according `expectSingleResult` option) in the payload of produced message is subscribed.
+The framework can subscribe to such a payload automatically (essentially `flatMap`) when splitter and `FluxMessageChannel` are used downstream.
+Otherwise it is target application responsibility to subscribe into a polled publishers in downstream endpoints.
+
+With Java DSL such a channel adapter could be configured like:
+
+====
+[source, java]
+----
+@Bean
+public IntegrationFlow reactiveMongoDbFlow(ReactiveMongoDatabaseFactory mongoDbFactory) {
+    return IntegrationFlows
+            .from(MongoDb.reactiveInboundChannelAdapter(mongoDbFactory, "{'name' : 'Name'}")
+                            .entityClass(Person.class),
+                    c -> c.poller(Pollers.fixedDelay(1000)))
+            .split()
+            .channel(c -> c.flux("output"))
+            .get();
+}
+----
+====

src/reference/asciidoc/reactive-streams.adoc

@@ -134,5 +134,5 @@ A returned reactive type can be subscribed immediately if we are in one-way, fir
 
 Currently Spring Integration provides channel adapter (or gateway) implementations for <<./webflux.adoc#webflux,WebFlux>>, <<./rsocket.adoc#rsocket,RSocket>> and <<./mongodb.adoc#mongodb,MongoDb>>.
 Also an https://github.com/spring-projects/spring-integration-extensions/tree/master/spring-integration-cassandra[Apache Cassandra Extension] provides a `MessageHandler` implementation for the Cassandra reactive driver.
-More reactive channel adapters are coming, for example for https://r2dbc.io/[R2DBC], https://mongodb.github.io/mongo-java-driver-reactivestreams/[MongoDB], for Apache Kafka in https://github.com/spring-projects/spring-integration-kafka[Spring Integration Kafka] based on the `ReactiveKafkaProducerTemplate` and `ReactiveKafkaConsumerTemplate` from https://spring.io/projects/spring-kafka[Spring for Apache Kafka] etc.
+More reactive channel adapters are coming, for example for https://r2dbc.io/[R2DBC], for Apache Kafka in https://github.com/spring-projects/spring-integration-kafka[Spring Integration Kafka] based on the `ReactiveKafkaProducerTemplate` and `ReactiveKafkaConsumerTemplate` from https://spring.io/projects/spring-kafka[Spring for Apache Kafka] etc.
 For many other non-reactive channel adapters thread pools are recommended to avoid blocking during reactive stream processing.

src/reference/asciidoc/whats-new.adoc

@@ -39,6 +39,5 @@ See <<./mongodb.adoc#mongodb-reactive-channel-adapters,MongoDB Reactive Channel
 The gateway proxy now doesn't proxy `default` methods by default.
 See <<./gateway.adoc/gateway-calling-default-methods,Invoking `default` Methods>> for more information.
 
-
 Internal components (such as `_org.springframework.integration.errorLogger`) now have a shortened name when they are represented in the integration graph.
 See <<./graph.adoc#integration-graph,Integration Graph>> for more information.