This commit ensures stream semantics (Flux vs Mono) are adhered to also
on the target side.

This commit adds a Decoder#decodeOne() method in order
to handle correctly the streaming versus one value
deserialization based on the type provided by the user.

For example, if a List parameter is provided in a controller
method, Jackson will be called once, while if the user provides
a Flux or an Observable parameter, Jackson will be called for
each element.

This commit replaces Reactive Streams converters for RxJava1 and
CompletableFuture with Reactor specific ones. The results in conversion
that preserves stream semantics, i.e. Mono vs Flux.

For example this is allowed:
Flux -> Observable
Mono -> Single
Mono -> CompletableFuture

This is not allowed:
Flux -> Single
Mono -> Observable
Flux -> CompletableFuture

As a result it is now possible to check through the ConversionService
if a target type to convert to is a stream of many or of one which is
useful for decoding purposes.

The commit also adds PublisherToFluxConverter to allow conversion from
raw Publisher to Flux. The reverse is not necessary since Flux is a
Publisher and it's a no-op conversion.

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.

AbstractRequestBodyPublisher.onDataAvailable improvement

When in state DATA_AVAILABLE if there are simultaneous invocations of
AbstractRequestBodyPublisher.RequestBodySubscription.request and
ReadListener.onDataAvailable, the first one will process the available
data, the second one should not throw an exception because thus it will
signal to web container that there are problems while there are not.

Refactored Undertow support to register a response listener only when
the body is written to, as opposed to registering it at startup. The
reason for this is that getting the response channel from the
HttpServerExchange commits the status and response, making it impossible
to change them after the fact.

Fixed issue #119.

Make AbstractResponseBodySubscriber.onWritePossible thread-safe

When there are simultaneous invocations of onWritePossible, only the
first one should succeed. This can happens when
AbstractResponseBodySubscriber.onNext and
WriteListener.onWritePossible() are called respectively by the
application and the web container.

This is a port of the following commit, adapted for Java 8+:
https://github.com/spring-projects/spring-framework/commit/89396ff01ff159aa7df18e332f3888cf9ce3dc20

This is a temporary measure for M1 as the dependency will go away once
issue #116 is addressed.

Make AbstractResponseBodySubscriber.onSubscribe thread-safe

When there are simultaneous invocations of onSubscribe, only the first one should succeed, the rest should cancel the provided subscriptions

Reactored Servlet 3.1 and Undertow request support
(AbstractResponseBodySubscriber) to use an internal state machine,
making thread-safity a lot easier.

 - AbstractResponseBodySubscriber now checks if the current state is
   expected before changing to a new state.
 - Included comments by @violetagg

Currently ResponseEntityResultHandler is ordered lower than
ResponseBodyResultHandler by default whch means a ResponseEntity
should not be picked by the ResponseBodyResultHandler.

However as it is easy to have both ResponseEntity and @ResponseBody
e.g. in @RestControler (or even by mistake) and in general it makes
sense for ResponseBodyResultHandler to explicitly recognize and
ignore the ResponseEntity return type.

We now also check the default content type if the content type is
application/octet-stream as we do today.

Uncommented failing test that now passes.

Before this commit only ResponseEntity with async body was supported,
e.g. ResponseEntity<Mono<String>>

This commit also adds suppport for an asyn wrapper around,
e.g. Mono<ResponseEntity<String>.

Introduce separate test classes for each base class in the hierarchy
above @ResponseBody and ResponseEntity result handlers.

Also start porting existing unit test cases for @ResponseBody and
ResponseEntity return value handlers.

CodecHttpMessageConverter is now suitable for SSE since it now
handles default content type.