Issue: SPR-17419

This commit fixes a memory leak in ServerSentEventHttpMessageWriter
that occurs when the input stream contains an error. Test added as well.

Issue: SPR-17419

Issue: SPR-17279

Issue: SPR-17086

Issue: SPR-16966

Issue: SPR-16516

This commits extends nullability declarations to the field level, formalizing the interaction between methods and their underlying fields and therefore avoiding any nullability mismatch.

Issue: SPR-15720

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 introduces 2 new @Nullable and @NonNullApi
annotations that leverage JSR 305 (dormant but available via
Findbugs jsr305 dependency and already used by libraries
like OkHttp) meta-annotations to specify explicitly
null-safety of Spring Framework parameters and return values.

In order to avoid adding too much annotations, the
default is set at package level with @NonNullApi and
@Nullable annotations are added when needed at parameter or
return value level. These annotations are intended to be used
on Spring Framework itself but also by other Spring projects.

@Nullable annotations have been introduced based on Javadoc
and search of patterns like "return null;". It is expected that
nullability of Spring Framework API will be polished with
complementary commits.

In practice, this will make the whole Spring Framework API
null-safe for Kotlin projects (when KT-10942 will be fixed)
since Kotlin will be able to leverage these annotations to
know if a parameter or a return value is nullable or not. But
this is also useful for Java developers as well since IntelliJ
IDEA, for example, also understands these annotations to
generate warnings when unsafe nullable usages are detected.

Issue: SPR-15540

Issue: SPR-15576

Issue: SPR-15464

This commit folds ServerHttpMessage[Reader|Writer] into its parent
HttpMessage[Reader|Writer] with the server methods pre-implemented
by default to be simple pass-through delegates.

CharSequenceEncoder now supports all MIME types, however since encoding
Flux<String> can overlap with other encoders (e.g. SSE) there are now
two ways to create a CharSequenceEncoder -- with support for text/plain
only or with support for any MIME type.

In WebFlux configuration we insert one CharSequenceEncoder for
text/plain (as we have so far) and a second instance with support for
any MIME type at the very end.

Issue: SPR-15374

Push the knowledge of what media types represent "streaming" down to
the Encoder level where knowledge is required (e.g. to encode a
JSON array vs a stream of JSON elements).

Instead of accepting List<Encoder|Decoder> and then look for the first
to support JSON, always expect a single JSON [Encoder|Decoder] and use
that unconditionally.

When writing use the nested ResolvableType instead of the Class of the
actual value which should better support generics.

Remove the SSE hint and pass "text/event-stream" as the media type
instead to serve as a hint. We are expecting a JSON encoder and using
it unconditionally in any case so this should be good enough.

ServerHttpEncoder and ServerHttpDecoder are HTTP-specific
specializations that can prepare encoding and decoding hints from
extra information available on the server side.

As a result Jackson2ServerHttpMessageReader is no longer needed.

The most common use case is specifying JSON views.

ServerResponse.BodyBuilder#hint(String, Object) allows to
specify response body serialization hints.

ServerRequest#body(BodyExtractor, Map) allows to specify
request body extraction hints.

Issue: SPR-15030

Issue: SPR-14899

Issue: SPR-14748

Issue: SPR-14557

Issue: SPR-14557

This commit avoid merging automatically the SSE stream with
Flux.never(). Since browsers automatically reconnect when
the HTTP connection is closed, it is now the user responsability
to optionally perform a concatWith(Flux.never()) on streams that
complete in order to avoid receiving the data multiple times on
client side.

The behavior with hot streams that never complete does not change.

Issue: SPR-14578

 - 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`.

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

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