Merge branch 'stable-2.15' into stable-2.16

* stable-2.15:
  Upgrade elasticsearch-rest-client to 6.5.1

Change-Id: I57d998de50909f2780115b822f53c0d1884df730
tree: 2c111292e21f45f456e90a6633bbc7ae582bf06d
  1. java/
  2. javatests/