ObjectToOptionalConverter needs to be marked as @UsesJava8
update documentation with latest forum infos

Forums will be shortly deactivated and questions should take place
in StackOverflow.
Defensively check javaUtilOptionalEmpty

Issue: SPR-11888
Defensively check javaUtilOptionalEmpty

Issue: SPR-11888
MBeanExportConfiguration's SpecificPlatform properly calls afterPropertiesSet

Also makes the nested SpecificPlatform enum public for Spring Boot to reuse it.

Issue: SPR-11877
WebSphereUowTransactionManager exposes SmartTransactionObject

Issue: SPR-11876
PathMatchingResourcePatternResolver's findPathMatchingResources needs to check for VFS before checking isJarResource

Issue: SPR-11887
ResourceUtils isFileURL detects "vfsfile" as a file system protocol (again)

Issue: SPR-11887
Refactor Version based ResourceResolvers

Prior to this commit, one of the available strategies for resolving
resources was the PrefixResourceResolver. Reconsidering the core goal of
this resolver and the FingerprintResourceResolver, we found that the
true core feature is versioning static resources application-wide.

This commit refactors both Resolvers by:
* adding a common Abstract Resolver for resolving resources based
on their version
* rename PrefixResourceResolver in VersionPrefixResourceResolver

One can add a VersionPrefixResourceResolver like this:

  List<ResourceResolver> resolvers = new ArrayList<ResourceResolver>();
  resolvers.add(new VersionPrefixResourceResolver("fooversion"));
  resolvers.add(new PathResourceResolver());

Issue: SPR-11871