As of 1.3.40, Kotlin now provides a kotlin-scripting-jsr223-embeddable
dependency which:
 - Fixes classloading related issues
 - Provides out of the box JSR 223 support
 - Is compatible with Spring Boot Fat Jar mechanism

This commit updates Spring Framework tests and documentation accordingly.

Closes gh-23165

Includes upgrade to Hibernate ORM 5.3.10 (for branch 5.1.x)

This commit also switches on kotlin-compiler-embeddable to
improve isolation of its dependencies

See gh-19975

This commit introduces the spring-core-coroutines module
in order to avoid referencing Kotlin code from Java one,
which is currently not supported by Eclipse.

During the build, spring-core-coroutines is merged into
spring-core, so this change is expected to have no impact
for end users.

This module contains functions accessible from Java via
the CoroutinesUtils class to adapt Coroutines and Deferred
instances to and from Mono.

See gh-19975

See gh-19975

Includes Mockito 2.25, Undertow 2.0.19, OpenPDF 1.2.10, JRuby 9.2.6.

This commit is the first part of a more complete Coroutines
support coming in Spring Framework 5.2. It introduces suspendable
Kotlin extensions for Mono based methods in WebFlux classes like
WebClient, ServerRequest, ServerResponse as well as a Coroutines
router usable via `coRouter { }`.

Coroutines extensions use `await` prefix or `AndAwait` suffix,
and most are using names close to their Reactive counterparts,
except `exchange` in `WebClient.RequestHeadersSpec`
which translates to `awaitResponse`.

Upcoming expected changes are:
 - Leverage `Dispatchers.Unconfined` (Kotlin/kotlinx.coroutines#972)
 - Expose extensions for `Flux` based API (Kotlin/kotlinx.coroutines#254)
 - Introduce interop with `CoroutineContext` (Kotlin/kotlinx.coroutines#284)
 - Support Coroutines in `ReactiveAdapterRegistry`
 - Support Coroutines for WebFlux annotated controllers
 - Fix return type of Kotlin suspending functions (gh-21058)

See gh-19975

Includes RxJava 2.2.6, Netty 4.1.33, Undertow 2.0.17.

Includes Hibernate ORM 5.4.1 for integration tests.

Includes OkHttp MockWebServer 3.12.1.

Includes JRuby 9.2.5 and Jetty Reactive HttpClient 1.0.2.

Includes JAX-WS 2.3.1, Awaitility 3.1.3, OkHttp 3.12, Woodstox 5.2.

Includes reordering of web dependency declarations.

Includes Hibernate ORM 5.1.16 and Hibernate Validator 6.0.13.

Includes Groovy 2.5.2, Undertow 2.0.12, Hibernate Validator 6.0.12.

Issue: SPR-17129

Issue: SPR-17124

Includes latest dependency updates (Mockito 2.21, Log4J 2.11.1, Hibernate ORM 5.3.4, Protobuf 3.6.1, JRuby 9.2, HtmlUnit 2.32, Selenium 3.14) for Spring Framework 5.1.

Issue: SPR-16388
Issue: SPR-16239

This commit introduces Protobuf support in WebFlux via dedicated
codecs.

Flux<Message> are serialized/deserialized using delimited Protobuf
messages with the size of each message specified before the message
itself. In that case, a "delimited=true" parameter is added to the
content type.

Mono<Message> are expected to use regular Protobuf message
format (without the size prepended before the message).

Related HttpMessageReader/Writer are automatically registered when the
"com.google.protobuf:protobuf-java" library is detected in the classpath,
and can be customized easily if needed via CodecConfigurer, for example
to specify protocol extensions via the ExtensionRegistry based
constructors.

Both "application/x-protobuf" and "application/octet-stream" mime types
are supported.

Issue: SPR-15776

Leverage https://github.com/jetty-project/jetty-reactive-httpclient
to add support for Jetty in WebClient via JettyClientHttpConnector.

Implemented with buffer copy instead of optimized buffer wrapping
because the latter hangs since Callback#succeeded doesn't allow
releasing the buffer and requesting more data at different times
(required for Mono<DataBuffer> for example).
See https://github.com/eclipse/jetty.project/issues/2429.

Issue: SPR-15092

Issue: SPR-17058

Prior to this commit, the generated POMs for Spring Framework modules
would contain unneeded/harmful information from the Spring Framework
build:

1. The BOM imports applied to each module by the dependency
management plugin, for example for Netty or Reactor Netty.
Spring should not export that opinion to its POMs.

2. The exclusion of "org.slf4:jcl-over-slf4j" from *all* dependencies,
which made the POMs much larger than necessary and suggested to
developers that they should exclude it as well when using all those
listed dependencies. In fact, only Apache Tiles currently brings that
transitively.

This commit removes that information from the POMs.
The dependencyManagement Gradle plugin is disabled for POM generation
and we manually resolve the dependency versions during the generation
phase.
The Gradle build is streamlined to exclude "org.slf4:jcl-over-slf4j"
only when necessary.

Issue: SPR-16893

...now that 0.8 is also listed in the Reactor bom.

Issue: SPR-16387

Issue: SPR-16387

Issue: SPR-16387

Issue: SPR-16470