From a user perspective, the main benefit from this upgrade is that the new
Lucene53Codec has disk-based norms. The elasticsearch directory has been fixed
to load these norms through mmap instead of nio.
Other changes include the removal of `max_thread_states`, the fact that
PhraseQuery and BooleanQuery are now immutable, and that deleted docs are now
applied on top of the Scorer API.
This change introduces a couple of `AwaitsFix`s but I don't think it should
hold us from merging.
Adrien Grand