Collapse ReactiveAdapter hierarchy into a single class that simply
delegates to functions for converting to/from a Publisher.

A private ReactorAdapter extension automaticlaly wraps adapted,  "raw"
Publisher's as Flux or Mono depending on the semantics of the target
reactive type.

Issue: SPR-14902

ReactiveTypeDescriptor is now a top-level type with named constructors.
It also contains and provides a getter for the actual type.

Issue: SPR-14902

Issue: SPR-14843

This commit adds support for RxJava 2 Completable,
Single, Observable and Flowable types (io.reactivex package).

Issue: SPR-14628

Issue: SPR-14159