Upgrade master to lucene 5.0 snapshot

This has a lot of improvements in lucene, particularly around memory usage, merging, safety, compressed bitsets, etc.

On the elasticsearch side, summary of the larger changes:
* API changes: postings API became a "pull" rather than "push", collector API became per-segment, etc.
* packaging changes: add ``lucene-backwards-codecs.jar`` as a dependency.
* improvements to boolean filtering: especially ensuring it will not be slow for SparseBitSet.
* use generic BitSet api in plumbing so that concrete bitset type is an implementation detail.
* use generic BitDocIdSetFilter api for dedicated bitset cache, so there is type safety.
* changes to support atomic commits
* implement ``Accountable.getChildResources`` (detailed memory usage API) for fielddata, etc
* change handling of IndexFormatTooOld/New, since they no longer extends CorruptIndexException