This commit adds support for reactive library types to be returned
directly from controller methods adapting them either to a
ResponseBodyEmitter (streaming) or DeferredResult (non-streaming).

The reactive libraries supported are the ones that can adapted to a
Reactive Streams Publisher through the ReactiveAdapterRegistry.

Issue: SPR-15365

Revise Javadoc on AsyncHandlerMethodReturnValueHandler to clarify its
main purpose is to prioritze custom async return value handlers ahead
of built-in ones. Also replace the interface from built-in handlers
which are prioritized already.

Remove DeferredResultAdapter and ResponseBodyEmitterAdapter --
introduced in 4.3 for custom async return value handling, since for
5.0 we will add built-in support for reactive types and the value of
these contracts becomes very marginal.

Issue: SPR-15365

* pr/1370:
  Polish javadoc

Closes gh-1370

Issue: SPR-15361

This change allows the functional WebFlux API to support natively
reactive types and also makes it possible for View implementations to
disable async attributes resolution if they want for example take
advantage of stream rendering.

It also makes AbstractView#getModelAttributes() asynchronous.

Issue: SPR-15368

Replace the overloaded "retrieve" methods with a single retrieve() +
ResponseSpec exposing shortcut methods (bodyToMono, bodyToFlux)
mirroring the ClientResponse shortcuts it delegates to.

Unlike exchange() however with retrieve() there is no access to other
parts of ClientResponse so ResponseSpec exposes additional shortcuts
for obtain ResponseEntity<T> or ResponseEntity<List<T>>.

Issue: SPR-15294

This commit makes a change to WebClient in oder to facilitate getting
the response body as a `Mono<Object>` or `Flux<Object>` without having
to deal with `ClientResponse`.

Specifically, this commit:

 - Adds `RequestHeaderSpec.retrieve` methods, next to `exchange`, that
 return the response body (and not a `ClientResponse`). Two convenience
 methods return the response body as `Mono` or `Flux`.
 - Adds ClientResponse.toRequestEntity to convert the ClientResponse
 into a RequestEntity.

Issue: SPR-15294

The main purpose is to specify nullability.

The ServerCodecConfigurer is now passed all the way into the
RequestMappingHandlerAdapter which automatically enables the same
defaults even without the Java config and provides extra flexibility
in passing message codec configuration for the future.

Restore the correct client-side default for whether StringDecoder
should split on new lines. It is true forthe server and false for the
client by default.

The regression was introduced in the recent refactoring:
https://github.com/spring-projects/spring-framework/commit/f8a21ab11bf1ecad7ac7866f280d42c70be48b8a#diff-0175d58138b2e8b2bec087ffe0495340

Replace the List<HandlerMethodArgumentResolver> with a dedicated
configurer that currently has one method accepting custom resolver
registrations.

This commit makes changes to WebClient and WebTestClient in oder to
limit setting the body according to HTTP method and also to facilitate
providing the request body as Object.

Specifically, this commit:

 - Moves methods that operate on the request body to a RequestBodySpec
 in both WebClient and WebTestClient, and rename them to `body`.
 These methods now just *set* the body, without performing
 an exchange (which now requires an explicit exchange call).
 - Parameterizes UriSpec in both WebClient and WebTestClient, so that
 it returns either a RequestHeadersSpec or a RequestBodySpec.

Issue: SPR-15394

This commit deprecates `AsyncRestTemplate` and related types
(`AsyncClientHttpRequestFactory` etc.) in favor of the Spring 5.0
`WebClient`.

Issue: SPR-15294

Issue: SPR-15393

Issue: SPR-14997

This commit removes docbook from the documentation toolchain and
instead makes use of asciidoctor to render the reference documentation
in HTML and PDF formats.

The main Gradle build has been refactored with the documentation tasks
and sniffer tasks extracted to their own gradle file in the "gradle"
folder.

A new asciidoctor Spring theme is also used to render the HTML5 backend.

Issue: SPR-14997

ControllerMethodResolver now also encapsulates initialization, storage,
and use of HandlerMethodArgumentResolver's by annotated method type.

Issue: SPR-14140

Extract controller method caches including associated code and
discovery of @ControllerAdvice components into a separate, package
private helper class (ControllerMethodResolver).

Issue: SPR-15247

Issue: SPR-15247

* pr/1278:
  Update Apache Licence 2.0 URL

Closes gh-1278

* pr/1333:
  Replace size() check with isEmpty()

Closes gh-1333

* pr/1334:
  Polish CollectionFactoryTests

Closes gh-1334

Spring JDBC not correctly processing Postgresql ?| and ?& operator