Issue: SPR-15590

This commit also changes "hibval5Version" to the more general "hibvalVersion" build variable, and includes dependency updates to Caffeine 2.5.1 and JRuby 9.1.9.

Issue: SPR-13482

Also removes JDK 9 build support for the time being since Gradle 3.5 still doesn't run on JDK 9.

Issue: SPR-15497

The MultipartHttpMessageWriter now directly encodes part header values
defaulting to UTF-8 and also specifies the charset in the
Content-Type header for the entire request.

This should work with something commonly used like Apache Commons
FileUpload which checks request.getCharacterEncoding() and uses it
for reading headers.

Closes gh-1409

Issue: SPR-14512

Issue: SPR-14908

Issue: SPR-15497

Issue: SPR-14546

This commit introduces reactive multipart support by adding a new
MultipartHttpMessageReader interface (with default methods) and a
SynchronossMultipartHttpMessageReader implementation based on
the Synchronoss NIO Multipart implementation
(https://github.com/synchronoss/nio-multipart).

Issue: SPR-14546

Issue: SPR-15463

Issue: SPR-14925

Issue: SPR-15462

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