Switch to JCR Oak to allow upgrade of lucene version

We are switching to JCR Oak that uses a higher lucene version.

But switching to Oak has some drawbacks:

• The repository format changed and we must provide a way to migrate (either
  migrate the existing repository or create a new one by reindexing)
• The JCR oak is asynchronous. So there may be some timing issues that are only visible at high load.
• Configuration changed significantly and not all is well documented

The incompatibility problem with the maven indexer that uses lucene in another version is solved with the maven indexer 6 snapshot version that uses now a shadowed lucene version.