Issue: SPR-17086

Issue: SPR-16966

Issue: SPR-16898

This commit ensures uniform support for decodeToMono across the various
byte and String related decoders.

Issue: SPR-16253

Beyond just formally declaring the current behavior, this revision actually enforces non-null behavior in selected signatures now, not tolerating null values anymore when not explicitly documented. It also changes some utility methods with historic null-in/null-out tolerance towards enforced non-null return values, making them a proper citizen in non-null assignments.

Some issues are left as to-do: in particular a thorough revision of spring-test, and a few tests with unclear failures (ignored as "TODO: NULLABLE") to be sorted out in a follow-up commit.

Issue: SPR-15540

Issue: SPR-15540

Issue: SPR-15540

This commit adds a ByteArrayDecoder and ByteArrayEncoder.

Issue: SPR-14557

Issue: SPR-14557

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

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

-> 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

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)

reactive.codec -> core.codec
reactive.io    -> util

These may very well not be the final locations. For now they simply
express that there are classes that belong somewhere in core, i.e.
they are lower level dependencies than web.

This commit introduces the following changes:
 - MessageToByteEncoder/Decoder renamed to Encoder/Decoder
 - JsonObjectEncoder/Decoder are now used directly in
   JacksonJsonEncoder/Decoder
 - Codec uses MimeType instead of MediaType since they
   are not specific to HTTP
 - Default MimeType are now managed thanks to
   Encoder/Decoder#getSupportedMimeTypes()
 - AbstractEncoder/Decoder takes care of generic MimeType related behavior

https://github.com/spring-projects/spring-framework/wiki/Spring-Framework-Code-Style

This commit introduces the following changes:
 - Publisher -> Observable/Stream/etc. conversion is now managed
    in a dedicated ConversionService instead of directly in
    RequestBodyArgumentResolver and ResponseBodyResultHandler
 - More isolated logic that decides if the stream should be
    serialized as a JSON array or not
 - Publisher<ByteBuffer> are now handled by regular
   ByteBufferEncoder and ByteBufferDecoder
 - Handle Publisher<Void> return value properly
 - Ensure that the headers are properly written even for response
   without body
 - Improve JsonObjectEncoder to autodetect JSON arrays

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.