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

There is really no need for a result handler dedicated to a void
return value and it's actually problematic to have it.

Each result handler treats void as necessary. For an @ResponseBody
method it means an empty body. For view resolution it means no specific
value was returned and we should procede with selecting a default view
name. Having a dedicated void result handler can interfere with this
especially since view resolution needs to be last in order.

At the same time there are cases when no result handling is needed
and the response is fully handled within the HandlerAdapter. This is
the case with WebHandler and the SimpleHandlerAdapter. For that case
we simply return mono.then(aVoid -> Mono.empty()) which effectively
returns an empty Mono and no result handling follows. The
HandlerAdapter already says you can return no values at all if the
response is fully handled.

Issue: SPR-14159

Rename two classes each adapting to WebHandler to avoid confusing them:
1. HttpWebHandlerAdapter adapts from the low level HttpHandler to any
WebHandler (e.g. DispatcherHandler).
2. SimpleHandlerAdapter adapts the plain WebHandler for use within the
DispatcherHandler.

This commit also fixes an issue in WebHttpHandlerBuilder to ensure that
WebExceptionHandler's are inserted before and not after WebFilter's.

This commit replaces the temporary SimpleUrlHandlerMapping with
a full-featured one.

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

WebServerExchange -> ServerWebExchange

Follows the same convention as in the http package also better allowing
the possibility for a client equivalent in the future.

WebToHttpHandlerBuilder -> WebHttpHandlerBuilder
WebToHttpHandlerAdapter -> WebHttpHandlerAdapter

More consistent with Spring conventions.

Introduce adapter and handler sub-packages under web.server following a
review prompted by the addition of the session package and the package
cycle it brought in based on dependency on session.WebSessionManager.

We use it only for Reactor Net support now.

-> reactor.Mono : reactor.core.publisher.Mono
-> reactor.Flux : reactor.core.publisher.Flux
-> reactor.Processors : reactor.core.publisher.Processors
-> reactor.Timers : reactor.core.timer.Timers
-> reactor.Subscribers : reactor.core.subscriber.Subscribers

This commit brings back the writeHeaders method on ServerHttpResponse
that was once added (2a6a4f) and then removed (9c7151).

This version is a little simpler since writeHeaders doesn't explicitly
flush/send headers which runtimes are expected to do by default.
Instead the main purpose of writeHeaders now is to ensure changes made
via HttpHeaders are applied to the underlying runtime response at some
point and we now do that once at the very end.

This approach provides the most flexibility (vs keeping HttpHeaders in
sync) because it allows a full and consistent set of mutative
operations for both headers and cookies (to be added) regardless of the
API exposed by the underlying runtime.

This change adds a WebServerExchange and updates all contracts at the
the same level (i.e. org.springframework.web.server) as well as the
org.springframework.web.reactive level to use it so that all
framework-related code will have access to server-side processing
features such as request attributes (and others to come).

This change separates out lower-level HTTP adapter code from the more
(framework-specific) HTTP processing into a separate package under
org.springframework.web.server (not under org.springframework.http).

Flux and Mono are used both for implementation and exposed at API
level to express 1 versus N semantic and to provide default Rx
operators:
- Flux<T> for multiple values Publisher (issue #48)
- Mono<T> for single value Publisher (issue #50)
- Mono<Void> for Publisher with no value (issue #49)

ServerHttpResponse implementations now immediately propagate
HttpHeaders changes as they so there is no need to call applyHeaders().

The writeHeaders from ServerHttpResponse is also removed. RxNetty and
Reactor Net both support implicitly completing if the handler
completes without explicitly writing the headers or the response body.

It brings along Servlet imports.

The web related code is now under org.springframework.web.reactive.
This is parallel to org.springframework.web (the top-level package of
spring-webmvc).

This commit introduces "reactive" sub-interfaces of the HttpMessage
interface found in the Spring Framework.

This commit updates HandlerAdapter#handle() to return HandlerResult
instead of Publisher<HandlerResult>. A new SimpleHandlerResultHandler
class has been introduced for handlers returning Publisher<Void>.

Just enough for a test with an @ResponseBody method that accepts an
@RequestParam String arg and returning Publisher<String> or String.

See RequestMappingIntegrationTests.

This commit adds support for simple URL handler mapping (exact path
match) and an adapter for the HttpHandler interface to be used to
handle the request.

The SimpleUrlHandlerMappingIntegrationTests then maps the URLs
"/foo" and "/bar" to two different handlers.