Latest dependency updates (Tomcat 8.5.14, Jetty 9.4.4, OkHttp 3.7, POI 3.16, Rome 1.7.2, EhCache 2.10.4)

The RxNetty runtime support is not meant to be officially supported and
should be restricted to testing purposes only.

Issue: SPR-15444

This commit introduces an explicit dependency on the latest JUnit
Vintage TestEngine so that it overrides whatever is bundled by
default in IntelliJ IDEA.

This allows for a more seamless upgrade to newer versions of JUnit 5 and
the JUnit Platform before IDEA has been officially updated to support
those versions, while simultaneously allowing developers to continue
to execute JUnit 4 based tests within IDEA.

Issue: SPR-15318

This commit introduces an explicit dependency on the latest JUnit
Platform Launcher API so that it overrides whatever is bundled by
default in IntelliJ IDEA.

This allows a more seamless upgrade to newer versions of JUnit 5 and
the JUnit Platform before IDEA has been officially updated to support
those versions.

This commit enables the `-use` javadoc flag so that class usage pages
are included in the generated JavaDoc.

Issue: SPR-15427

Issue: SPR-15383

Issue: SPR-14923

Includes upgrade to HtmlUnit 2.26 and Selenium 3.3.1.

Issue: SPR-14512

Includes dependency updates (JavaMail 1.6 RC1, RxJava 2.0.8, Tomcat 8.5.13, Undertow 1.4.12, XNIO 3.4.5)

Issue: SPR-14512

This commit adds support for reactive library types to be returned
directly from controller methods adapting them either to a
ResponseBodyEmitter (streaming) or DeferredResult (non-streaming).

The reactive libraries supported are the ones that can adapted to a
Reactive Streams Publisher through the ReactiveAdapterRegistry.

Issue: SPR-15365

Issue: SPR-15361

This commit removes docbook from the documentation toolchain and
instead makes use of asciidoctor to render the reference documentation
in HTML and PDF formats.

The main Gradle build has been refactored with the documentation tasks
and sniffer tasks extracted to their own gradle file in the "gradle"
folder.

A new asciidoctor Spring theme is also used to render the HTML5 backend.

Issue: SPR-14997

Latest dependency updates (RxJava 1.2.9, JiBX 1.3.1, H2 1.4.194, JBoss Logging 3.3.1, WebJars Locator 0.32-1)

Issue: SPR-14925

Currently the BOM versions are:

* reactor-core 3.0.6.BUILD-SNAPSHOT
* reactor-netty 0.6.2.BUILD-SNAPSHOT

This commit fixes as well a few deprecations in reactor-core.

Issue: SPR-15147

Includes Log4J 2.8.1 and Selenium 3.2 as well.

Issue: SPR-15319

Issue: SPR-15301

Includes updates to Caffeine 2.4, Jetty 9.4.2, Undertow 1.4.11, RxJava 1.2.7 and Groovy 2.4.9.

Issue: SPR-15207