DATACMNS-836 - Add reactive repository support.

We now expose reactive interfaces to facilitate reactive repository support in store-specific modules. Spring Data modules are free to implement their reactive support using either RxJava 1 or Project Reactor (Reactive Streams). We expose a set of base interfaces:

* `ReactiveCrudRepository`
* `ReactiveSortingRepository`
* `RxJava1CrudRepository`
* `RxJava1SortingRepository`

Reactive repositories provide a similar feature coverage to blocking repositories. Reactive paging support is limited to a `Mono<Page>`/`Single<Page>`. Data is fetched in a deferred way to provide a paging experience similar to blocking paging.

A store module can choose either Project Reactor or RxJava 1 to implement reactive repository support. Project Reactor and RxJava types are converted in both directions allowing repositories to be composed of Project Reactor and RxJava 1 query methods. Reactive wrapper type conversion handles wrapper type conversion at repository level. Query/implementation method selection uses multi-pass candidate selection to invoke the most appropriate method (exact arguments, convertible wrappers, assignable arguments).

We also provide ReactiveWrappers to expose metadata about reactive types and their value multiplicity.

pom.xml

@@ -105,6 +105,15 @@
 			<optional>true</optional>
 		</dependency>
 
+		<!--  RxJava -->
+
+		<dependency>
+			<groupId>io.reactivex</groupId>
+			<artifactId>rxjava</artifactId>
+			<version>${rxjava}</version>
+			<optional>true</optional>
+		</dependency>
+
 		<!--  Querydsl -->
 
 		<dependency>