- 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`.

Issue: SPR-14646

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

Prior to this commit, exceptions thrown by the
`HandlerMethodArgumentResolver` would be wrapped into
`IllegalStateException`s.

This commit makes sure that a DEBUG log is written with the relevant
information and that the root cause is not wrapped into another
exception, so that the appropriate `ExceptionHandler` can be used to
deal with this error.

Issue: SPR-14618

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.

DataSourceUtils moved to main core.io.buffer package.
Consistently named Jackson2JsonDecoder/Encoder and Jaxb2XmlDecoder/Encoder.
Plenty of related polishing.

This commit refactors the StringEncoder to a CharSequenceEncoder, in
order to support StringBuilders, Groovy GStrings, etc.

Issue: https://github.com/spring-projects/spring-reactive/issues/120

Issue: SPR-14159

This commit changes the reactive flushing mechanism to use a newly
introduced writeAndFlushWith(Publisher<Publisher<DataBuffer>>) on
ReactiveHttpOutputMessage instead of using the FlushingDataBuffer.

Issue: https://github.com/spring-projects/spring-reactive/issues/125

Issue: SPR-14479

Consistently use constructor-based instantiation instead of Class.newInstance / BeanUtils.instantiate

Issue: SPR-14486

Issue: SPR-14484

Ensure type-level Javadoc in every class, comply with guidelines for
80 char on Javadoc, and minor polish.

Moved ResponseBodyProcessor creation from constructor to
writeWithInternal(), in preparation of supporting both
Publisher<DataBuffer> as well as Publisher<Publisher<DataBuffer>>.

In preparation of supporting both Publisher<DataBuffer> and
Publisher<Publisher<DataBuffer>> as response body, moved
RequestBodyPublisher and ResponseBodyProcessor into
ServletServerHttpRequest and ServletServerHttpResponse respectively.

Refactored Publisher<Void> in AbstractResponseBodyProcessor into
separate ResponseBodyWriteResultPublisher.

This commit adds the required infrastructure to build HTTP requests as
well as extracting relevant information from HTTP responses using the
RxJava 1.x API, where Observable and Single don't extend Publisher.

This commit refactors the `ClientHttpRequestFactory` into an
`ClientHttpConnector` abstraction, in order to reflect that
`ClientHttpRequest`s only "exist" once the client is connected
to the origin server.

This is why the HTTP client is now callback-based, containing all
interactions with the request within a
`Function<ClientHttpRequest,Mono<Void>>` that signals when it's done
writing to the request.

The `ClientHttpRequest` contract also adopts `setComplete()`
and promotes that method to the `ReactiveHttpOutputMessage` contract.

This commit also adapts all other APIs to that change and fixes a few
issues, including:

* use `HttpMessageConverter`s instead of `Encoders`/`Decoders`
* better handle type information about request content publishers
* support client cookies in HTTP requests
* temporarily remove the RxNetty client support

Reactor's `DependencyUtils` has been renamed to `Converters` and
all the `from` converter methods have been disambiguated to
`fromPublisher`, `toPublisher`.

The state machine is refactored in order to solve various concurrency
issues.

This commit adds support for handling an empty request body with both
HttpEntity where the body is not required and with @RequestBody where
the body is required depending on the annotation's required flag.

If the body is an explicit type (e.g. String, HttpEntity<String>) and
the body is required an exception is raised before the method is even
invoked or otherwise the body is passed in as null.

If the body is declared as an async type (e.g. Mono<String>,
HttpEntity<Mono<String>>) and is required, the error will flow through
the async type. If not required, the async type will be passed with no
values (i.e. empty).

A notable exception is rx.Single which can only have one value or one
error and cannot be empty. As a result currently the use of rx.Single
to represent the request body in any form effectively implies the body
is required.

Before this change decodeToMono always created a StringBuilder to
aggregate resulting in an "" (empty string) rather than an empty
Mono for an empty input stream.

Now we aggregate in the DataBuffer instead and then decode to String.

The RequestBodyArgumentResolver has been refactored to have a shared
base class and tests with the new HttpEntityMethodArgumentResolver.

An HttpEntity argument is not expected to have an async wrapper because
the request headers are available immediately. The body however can be
asynchronous, e.g. HttpEntity<Flux<String>>.

* update bean validation API version
* MethodParameter now validates indices, see 39e3f2eb
* `getCharSet()` is now `getCharset()`