Simplify getAdapterFrom/To into a single getAdapter method that looks
for an exact match by type first and then isAssignableFrom.

Also expose shortcut methods in ReactiveAdapter to minimize the need
for access to the ReactiveTypeDescriptor.

Issue: SPR-14902

Collapse ReactiveAdapter hierarchy into a single class that simply
delegates to functions for converting to/from a Publisher.

A private ReactorAdapter extension automaticlaly wraps adapted,  "raw"
Publisher's as Flux or Mono depending on the semantics of the target
reactive type.

Issue: SPR-14902

ReactiveTypeDescriptor is now a top-level type with named constructors.
It also contains and provides a getter for the actual type.

Issue: SPR-14902

Issue: SPR-14880

Issue: SPR-14880

Issue: SPR-14800

 - ScriptedSubscriber has been renamed to Verifier
 - The Publisher is passed to create() instead of verify()
 - No more need to specify the generic type explicitly
 - Version is now sync with reactor-core

Issue: SPR-14800

Issue: SPR-14304

Issue: SPR-14843

Reactor recently added the `ScriptedSubscriber` in its new
`reactor-addons` module. This `Subscriber` revissits the previous
`TestSubscriber` with many improvements, including:

* scripting each expectation
* builder API that guides you until the final verification step
* virtual time support

This commit refactor all existing tests to use this new
infrastructure and removed the `TestSubscriber` implementation.

Issue: SPR-14800

This commit adds a ByteArrayDecoder and ByteArrayEncoder.

Issue: SPR-14826

DefaultDataBuffer.grow(int):
- Copy only the remaining data
- Update readPosition/writePosition

This commit adds the necessary infrastructure for the support of HTTP
Range requests. The new `ResourceRegionEncoder` can write
`ResourceRegion` objects as streams of bytes.

The `ResourceRegionEncoder` relies on an encoding hint
`BOUNDARY_STRING_HINT`. If present, the encoder infers that multiple
`ResourceRegion`s should be encoded and that the provided boundary
String should be used to separate ranges by mime boundaries.
If that hint is absent, only a single resource region is encoded.

Issue: SPR-14664

Issue: SPR-14557

(cherry picked from commit ace25d49)

Issue: SPR-14718

Added readableChannel() to Resource, which returns a
java.nio.ReadableByteChannel. The default implementation uses
Channels.newChannel() to create a channel based on what is returned from
getInputStream(). Subclasses have more effecient, file-based
implementations.

Issue: SPR-14698

Issue: SPR-14557

This commit adds a "spring-context-indexer" module that can be added to
any project in order to generate an index of candidate components defined
in the project.

`CandidateComponentsIndexer` is a standard annotation processor that
looks for source files with target annotations (typically `@Component`)
and references them in a `META-INF/spring.components` generated file.

Each entry in the index is the fully qualified name of a candidate
component and the comma-separated list of stereotypes that apply to that
candidate. A typical example of a stereotype is `@Component`. If a
project has a `com.example.FooService` annotated with `@Component` the
following `META-INF/spring.components` file is generated at compile time:

```
com.example.FooService=org.springframework.stereotype.Component
```

A new `@Indexed` annotation can be added on any annotation to instructs
the scanner to include a source file that contains that annotation. For
instance, `@Component` is meta-annotated with `@Indexed` now and adding
`@Indexed` to more annotation types will transparently improve the index
with additional information. This also works for interaces or parent
classes: adding `@Indexed` on a `Repository` base interface means that
the indexed can be queried for its implementation by using the fully
qualified name of the `Repository` interface.

The indexer also adds any class or interface that has a type-level
annotation from the `javax` package. This includes obviously JPA
(`@Entity` and related) but also CDI (`@Named`, `@ManagedBean`) and
servlet annotations (i.e. `@WebFilter`). These are meant to handle
cases where a component needs to identify candidates and use classpath
scanning currently.

If a `package-info.java` file exists, the package is registered using
a "package-info" stereotype.

Such files can later be reused by the `ApplicationContext` to avoid
using component scan. A global `CandidateComponentsIndex` can be easily
loaded from the current classpath using `CandidateComponentsIndexLoader`.

The core framework uses such infrastructure in two areas: to retrieve
the candidate `@Component`s and to build a default `PersistenceUnitInfo`.
Rather than scanning the classpath and using ASM to identify candidates,
the index is used if present.

As long as the include filters refer to an annotation that is directly
annotated with `@Indexed` or an assignable type that is directly
annotated with `@Indexed`, the index can be used since a dedicated entry
wil be present for that type. If any other unsupported include filter is
specified, we fallback on classpath scanning.

In case the index is incomplete or cannot be used, The
`spring.index.ignore` system property can be set to `true` or,
alternatively, in a "spring.properties" at the root of the classpath.

Issue: SPR-11890

This commit adds support for RxJava 2 Completable,
Single, Observable and Flowable types (io.reactivex package).

Issue: SPR-14628

Issue: SPR-14648

Issue: SPR-14645

Issue: SPR-14492

This commit also sets the test source encoding to UTF-8.

Issue: SPR-14547

This commit adds support for HTTP header field parameters encoding, as
described in RFC5987.
Note that the default implementation still relies on US-ASCII encoding,
as the latest rfc7230 Section 3.2.4 says that:

> Newly defined header fields SHOULD limit their field values to
  US-ASCII octets

Issue: SPR-14547

(cherry picked from commit 1932a9d7)

reactor.test.TestSubscriber will not be part of Reactor Core
3.0.0 since it needs to be refactored to fit all the needs
expressed by the users. It is likely to be back later in one
of the Reactor Core 3.0.x releases.

This commit anticipate this removal by temporarily copying
TestSubscriber in spring-core test classes. As soon as
the new TestSubscriber will be available in Reactor Core,
Spring Framework reactive tests will use it again.

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

This commit also drops the now-unused CUSTOM_COMPILATION test group and the outdated JavaVersion class.

Issue: SPR-10423

Issue: SPR-14509

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