Integrate Spring Data MongoDB Reactive repositories

[mp911de]

This PR integrates with Spring Data MongoDB reactive repositories.

Spring Data exposes reactive repositories using Project Reactor and RxJava types. Spring Data allows RxJava, Project Reactor or mixed repositories.

public interface ReactiveMongoRxJavaPersonRepository extends RxJavaCrudRepository<Person, String> {

}

public interface ReactiveMongoPersonRepository extends ReactiveCrudRepository<Person, String> {

}

public interface ReactiveMongoRxJavaPersonRepository extends RxJavaCrudRepository<Person, String> {

    Flux<Person> findByLastname(String lastname);

    Single<Person> findByFirstnameAndLastname(String firstname, String lastname);
}

---

Spring Data MongoDB Branch: issue/DATAMONGO-1444
Requirements: MongoDB ReactiveStreams driver 1.2.0, Project Reactor 2.5.0.M4
Artifact coordinates:

<dependency>
    <groupId>org.springframework.data</groupId>
    <artifactId>spring-data-mongodb</artifactId>
    <version>2.0.0.DATAMONGO-1444-SNAPSHOT</version>
</dependency>

Please note that the project fails with HTTP/1.1 500 Internal Server Error and Caused by: org.springframework.web.server.UnsupportedMediaTypeStatusException: Request failure [status: 415, reason: "Content type 'application/json' not supported"]

PostgresPersonRepository and CouchbasePersonRepository are based on Project Reactor 2.5.0.M3 and need to be migrated from .after() to .then().