Merge pull request #1168 from lucamilanesio/bump-to-lucene-5.5.2
Bump to Lucene 5.5.2
The new code will create Lucene indices in a new directory named after codec and index version.
This provides for easy and safe up- and downgrades. But it also means that the old indices will
stick around on disk. What this version is missing is a kind of "garbage collection" deleting old, unused
indices when they are no longer needed. That task needs to be done manually currently. We should
at leas at some point provide a script for it.