This commit adds a new chain-based, interception contract to be used
with `WebClient`. This is the HTTP client equivalent of the `WebFilter`
contract already implemented in web reactive server.

A `ClientHttpRequestInterceptor` implementation can transform the
outgoing HTTP request (method, URI or headers) before delegating it to
the next interceptor in the chain, or bypass the request processing
altogether and return a (cached) HTTP response.

Issue: SPR-14502

This commit also removes the ServerHttpResponse parameter, not
needed here since it is possible to override the write() method.

Issue: SPR-14557

This new `HttpMessageWriter` leverages the `ResourceRegionEncoder` to
write `ResourceRegion` to HTTP responses, thus supporting HTTP Range
requests.

Whenever possible, this message writer uses the zero copy support for
single range requests.

This `HttpMessageWriter` is never used directly, but is used as a
delegate by the `ResourceHttpMessageWriter`. When provided with the
`BOUNDARY_STRING_HINT`, the `ResourceRegionHttpMessageWriter`
adapts its behavior in order to write a single/multiple byte ranges.

Issue: SPR-14664

This commit adds the necessary infrastructure for the support of HTTP
Range requests. The new `ResourceRegionEncoder` can write
`ResourceRegion` objects as streams of bytes.

The `ResourceRegionEncoder` relies on an encoding hint
`BOUNDARY_STRING_HINT`. If present, the encoder infers that multiple
`ResourceRegion`s should be encoded and that the provided boundary
String should be used to separate ranges by mime boundaries.
If that hint is absent, only a single resource region is encoded.

Issue: SPR-14664

This commit makes it possible, in addition to provide hints, to
perform additional operations with the request and the response
at ServerHttpMessageReader/Writer level.

AbstractServerHttpMessageReader/Writer now provide
convenient beforeRead/beforeWrite abstract methods for such need.

Issue: SPR-14557

Issue: SPR-14557

Issue: SPR-14538

Issue: SPR-14682

Issue: SPR-14726

Move the AbstractServletHttpHandlerAdapterInitializer together with
the other two base classes in spring-web-reactive. Since the
interface is in the web package, this avoids a package cycle.
Also add a mention in the reference.

Issue: SPR-14713

This commit introduces three new WebApplicationInitializers for use with
Spring Web Reactive:

 - The AbstractServletHttpHandlerAdapterInitializer registers a
 ServletHttpHandlerAdapter that wraps a user-provided HttpHandler.

 - The AbstractDispatcherHandlerInitializer registers a
 ServletHttpHandlerAdapter that wraps a DispatcherHandler (or any
 WebHandler). The handler is provided with an application context.

 - The AbstractAnnotationConfigDispatcherHandlerInitializer is a
 subclass of the above, creating an
 AnnotationConfigApplicationContext based no provided @Configuration
 classes.

Issue: SPR-14713

Issue: SPR-14693

Issue: SPR-14693

Issue: SPR-14669

Rather than setting the status to 503 directly from the timeout
interceptor which no longer seems to work reliably with Servlet
containers like Jetty even performing an additional ERROR dispatch back
to the original URL, we know rather set the DeferredResult to an
AsyncTimeoutException, which results in a dispatch and standard
handling within Spring MVC. This should be a more reliable way of
dealing with timeouts.

Issue: SPR-14669

Issue: SPR-14643

Issue: SPR-14557

Issue: SPR-14557

Since SPR-14522, the web reactive framework supports checkNotModified
features. This commit aligns the existing MVC infrastructure with
web reactive's behavior.

Because of the new Servlet 3.0 baseline, some constraints
aren't relevant anymore and duplicate code has been removed in
`HttpEntityMethodProcessor`.

Issue: SPR-14659

As per specification "The Subscription MUST allow the Subscriber to
call Subscription.request synchronously from within onNext or
onSubscribe". With the current implementation if Subscription.request
is called more than once when Subscriber.onSubscribe ISE will be
thrown - java.lang.IllegalStateException: DEMAND.
With this fix the implementation will not throw ISE and will allow
many invocations of Subscription.request when
Subscriber.onSubscribe.

Issue: SPR-14522

Issue: SPR-14522

This commit avoid merging automatically the SSE stream with
Flux.never(). Since browsers automatically reconnect when
the HTTP connection is closed, it is now the user responsability
to optionally perform a concatWith(Flux.never()) on streams that
complete in order to avoid receiving the data multiple times on
client side.

The behavior with hot streams that never complete does not change.

Issue: SPR-14578

 - Renamed SseEvent to ServerSentEvent to make the name less redundant.
 - ServerSentEvent is now immutable, having a builder to create new instances.
 - Realigned the class properties to more closely match the events
   described in the spec, so that `reconnectTime` becomes `retry`, and
   `name` becomes `event`.

Operators.getAndSub was removed in Reactor 3.0.1, this commit copies the
implementation over to AbstractRequestBodyPublisher, which used it.

Issue: SPR-14646

(cherry picked from commit 37670924)

Issue: SPR-14635

This commit updates the instructions on getting started with
Spring Web Reactive and also updates constructors and setters to
streamline the getting started procedure.

Issue: SPR-14640