Issue: SPR-14159

Issue: SPR-14158

Issue: SPR-14158

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-14492

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.

https://github.com/eclipse/jetty.project/issues/730

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

Moved log4j.properties from the main source tree to the test tree.

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 replaces the current helper methods in
RequestMappingIntegrationTests with generic helper methods to perform
HTTP GET and POST requests.

This results in more transparent code that shows the exact HTTP
inputs and outputs and is also more flexible to change for
variations in testing.

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>>.