The renaming makes it clear this exception is for use where 400 error
applies within a Spring web application where the error may be
associated with a MethodParameter, a BindingResult, and so on.

There is no need for BadRequestStatusException which can be expressed
with ResponseStatusException(HttpStatus.BAD_REQUEST, "reason").

@ResponseBody Resource support

This commit wraps up the previous commits:

 - It uses HttpMessageConverter in the web.reactive.server package instead of Encoder/Decoder.
 - It introduces tests for the Resource @ResponseBodies.

This commit introduces a reactive version of the HttpMessageConverter.
During the implementation of zero-copy support, it became apparent that
it was ueful to have a common abstraction between client and server that
operated on HttpMessages rather than DataBuffers.

Two HttpMessageConverter implementations are provided:
 - The CodecHttpMessageConverter, based on Encoder/Decoder.
 - The ResourceHttpMessageConverter, using zero-copy if available.

This commit introduces support for zero-copy file transfers in the HTTP
response, through the ZeroCopyHttpOutputMessage subinterface of
ReactiveHttpOutputMessage.

This commit introduces two DataBuffer improvements:

 - The capability to read a Flux<DataBuffer> from an input stream or
   channel.

 - The capability to limit a Publisher<DataBuffer> to publish up until a
   given maximum byte count.

This commit replaces use of the existing ServletException-based
exceptions with the new ones from ~.web.server.

No need to translate to response status codes any more and no need for
an exception handling mechanism at this level.

Now that we have a CompositeContentTypeResolverBuilder with built-in
defaults, we switch to those defaults in places where a
ContentTypeResolver is used.

This is the equivalent of the existing ContentNegotiationManager +
ContentNegotiationManagerFactoryBean

Rename to MappingContentTypeResolver and replace "fileExtension" with
"key" in methods to be more general (e.g. file extension or query
param). Also switch from List to Set for the return values.

This commit adds RequestMappingInfoHandlerMapping and
RequestMappingHandlerMapping with support equivalent to that in
spring-webmvc.

A pretty complete equivalent to the same in spring-webmvc except for
CORS checks, and custom HTTP methods. Another notable difference is
that the "params" condition works on query params strictly.

A starting point for an alternative to the existing
ContentNegotiationStrategy but for use with ServerWebExchange.

Consistent initialization of HttpHeaders and the cookies map.
 - allow sub-classes to create instance
 - make unmodifiable

This commit adds AbstractHandlerMethodMapping, a starting point for
AbstractHandlerMapping, and HttpRequestPathHelper with a similar
purpose to UrlPathHelper but based with ServerWebExchange as input.

The new package is parallel to the "mvc" package under spring-webmvc
and a place to hold the various programming model styles.

- correct name of HttpHandlerHandlerAdapter to WebHandlerHandlerAdapter
- shorten SimpleHandlerResultHandler to SimpleResultHandler
- add HandlerResult constructor without Model
- update tests