Prior to this commit, the Spring Framework build would partially use the
dependency management plugin to import and enforce BOMs.

This commit applies the dependency management plugin to all Java
projects and regroups all version management declaration in the root
`build.gradle` file (versions and exclusions).

Some versions are overridden in specific modules for
backwards-compatibility reasons or extended support.

This commit also adds the Gradle versions plugin that checks for
dependency upgrades in artifact repositories and produces a report; you
can use the following:

    ./gradlew dependencyUpdates

This commit adds SPI interfaces to support reactive transactions
through spring-tx with optional dependencies to Project Reactor and
supportive implementations for TransactionalOperator and
AbstractReactiveTransactionManager.

Closes gh-20361

Issue: SPR-17446

Issue: SPR-16685

Upgrade to JAXB 2.3, JAX-WS 2.3, Annotations 1.3.1, Interceptor 1.2.1.
Also includes Log4J 2.9.1 and Asciidoctor 1.5.6.

Latest dependency updates (POI 3.17, Rome 1.8, EhCache 3.4, Caffeine 2.5.6, RxJava 2.1.4, Tomcat 8.5.21, JRuby 9.1.13, Rhino 1.7.7.2)

The main `build.gradle` file contains now only the common build
infrastructure; all module-specific build configurations have
been moved to their own build file.

Issue: SPR-15885