Issue: SPR-14546

Issue: SPR-15508

Issue: SPR-14546

With this commit, ServerCodecConfigurer is now exposed as a bean in
order to be provided to DefaultServerWebExchange via
WebHttpHandlerBuilder and HttpWebHandlerAdapter. This allows
DefaultServerWebExchange to get configured codecs for reading form or
multipart requests.

Issue: SPR-14546

Issue: SPR-15318

Issue: SPR-15350

MockServerHttpRequest and MockServerHttpResponse now extend the same
abstract base classes that server-specific implementations do and
therefore approximate their behavior more closely.

As an immediate consequence MockServerHttpRequest is read-only after
it is created. Instead it now exposes static builder methods similar
to those found in RequestEntity. This enforces more strictness as well
as recycling of requests in tests and provides nicer builder methods.

To simplify tests DefaultServerWebExchange now offers a constructor
with just a request and response, and automatically creating a
DefaultWebSessionManager.

The spring-test module now also contains client-side reactive mock
request and response implementations. The mock client request extends
the same AbstractClientHttpRequest as client-specific implementations
do. There is no abstract base class for client responses.

Issue: SPR-14590

ServerWebExchange now provides access to "requestParams" as a
MulitValueMap with query parameters and form data combined.

The combined map is then used for the params condition of
@RequestMapping purposes () and also for @RequestParam arguments.

Issue: SPR-15000

The method to access the Principal from the ServerWebExchange is now
a Mono<Principal> (rather than Optional<Principal>).

There is also support for Principal as a controller method argument.

Issue: SPR-14680, SPR-14865

The ServerWebExchange now has a getFormData() method that delegates to
FormHttpMessageReader for the parsing and then caches the result so
it may be used multiples times during request processing.

Issue: SPR-14541

ServerWebExchange now has a getPrincipal method and along with that a
ServerWebExchangeDecorator that can be used to wrap the exchange in
order to return the authenticated user.

Issue: SPR-14680

Issue: SPR-14522

Issue: SPR-14522

The DefaultWebSessionManager now uses Mono.defer to protect the call
to getSession from parsing session cookies immediately. This allows
pre-initializing the Mono<WebSession> upfront vs using a lock.

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.

This commit adds initial support for a maintaining a server-side
session with attributes across HTTP requests. The WebSession
abstraction can be accessed via WebServerExchange from a WebFilter or
the target WebHandler.

The session sub-package contains additional abstractions for creating
and managing sessions providing a basis for extensibility (e.g. Spring
Session). Those include WebSessionManager, SessionIdStrategy, and
SessionStore along with a cookie-based session id strategy and an
in-memory session store in use by default.

Note that the current API does not provide a way to invalidate or
re-create the session from server side code.

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)

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 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.

Introduce http and dispatch packages under org.springframework.web to
separate HTTP-specific adapter code from higher level web framework
code. Move classes into their respective locations.

This commit introduces HTTP request and response abstractions along
with Servlet-based implementations similar to the ones in the http
package of spring-web but using Reactive Streams.

In turn HttpHandler now accepts the request and response types and
returns Publisher<Void> that reflects the end of handling.

The write method on the response also returns Publisher<Void> allowing
deferred writing. At the moment however the underlying Servlet 3.1
support only supports a single publisher after which the connection
is closed.

Only simple byte[] is supported for reading and writing.