This commit introduces a new method on WebClient: filter, which takes an
filter function, and returns a (filtered) WebClient.

Issue: SPR-14961

- Add 4xx/5xx status code check to ClientResponse.bodyToMono and bodyToFlux.
- Removed WebClient.retrieveMono and retrieveFlux.

Issue: SPR-14977

Issue: SPR-14975

ChannelFutureMono renamed to FutureMono

In the latest reactor-netty there is renaming
of ChannelFutureMono class to FutureMono. Thus
build fails on the latest master.

Issue: SPR-14533

This commit adds the ability to serve Resources (static files) through a
RouterFunction. Two methods have been added to RouterFunctions: one that
exposes a given directory given a path pattern, and a generic method
that requires a lookup function.

Issue: SPR-14913

Issue: SPR-14467

(cherry picked from commit 0028b297)

Issue: SPR-14534

Issue: SPR-14969

Create a ReactorNettyCodec to hold the decoding and encoding function
and consumer along with a package-private sub-class that delegates to
StompDecoder and StompEncoder.

Issue: SPR-14531

Issue: SPR-14960

Issue: SPR-14958

Issue: SPR-12889

Issue: SPR-14926
Issue: SPR-12926

* getBeanDefinitionNames defensively returns a copy of the bean definition names array.
* copyConfigurationFrom provides an independent AutowireCandidateResolver instance and copies a ConversionService and dependency comparator configuration as well.
* findAutowireCandidates only considers a self reference fallback for direct dependency declarations, not as a collection element.

Issue: SPR-14897
Issue: SPR-14921
Issue: SPR-14965

Issue: SPR-14924

In general, web.util is supposed to remain lowest-level, not depending on any other web.* package. Since web.client also has a support package, a corresponding web.server.support package seemed appropriate for a helper class depending on ServerWebExchange in web.server itself.

The ResponseStatusException is now also supported in Spring MVC
through the ResponseStatusExceptionResolver.

Issue: SPR-14895

The recent refactoring lead to
java.lang.NoClassDefFoundError: io/reactivex/Completable

where only RxJava 1 is in the classpath. Most likely due to the
lack of prefix in Completable::complete with rx package to avoid
the RxJava 2 reference.

Issue: SPR-13481

Issue: SPR-13188

Reactor Netty's `HttpServerResponse` has a new `sendFile` signature that
takes a `Path` instead of a `File`.

Issue: SPR-14916

Issue: SPR-14939

Issue: SPR-14937

Main differences with the Spring MVC original implementation:
 - Default redirect HTTP code is 303 See Other since we can assume
   all HTTP clients support HTTP 1.1 in 2016
 - No more http10Compatible property, use statusCode instead
 - By default the redirect is relative to the context path
 - A builder allow to set various properties if needed
 - In UrlBasedViewResolver, a Function<String, RedirectView>
   redirectViewProvider property allows to customize RedirectView
   instances in a flexible way

Issue: SPR-14534

Simplify getAdapterFrom/To into a single getAdapter method that looks
for an exact match by type first and then isAssignableFrom.

Also expose shortcut methods in ReactiveAdapter to minimize the need
for access to the ReactiveTypeDescriptor.

Issue: SPR-14902