* master: (27 commits)
  Check index sorting with no replica since we cannot ensure that the replica index is ready when forceMerge is called. Closes elastic#24416
  Docs: correct indentation on callout
  Build that java api docs from a test (elastic#24354)
  Move RemoteClusterService into TransportService (elastic#24424)
  Fix license header in WildflyIT.java
  Try not to lose stacktraces (elastic#24426)
  [DOCS] Update XPack Reference URL for 5.4 (elastic#24425)
  Painless: Add tests to check for existence and correct detection of the special Java 9 optimizations: Indified String concat and MethodHandles#ArrayLengthHelper() (elastic#24405)
  Extract a common base class to allow services to listen to remote cluster config updates (elastic#24367)
  Adds check to snapshot repository incompatible-snapshots blob to delete a pre-existing one before attempting to overwrite it.
  Added docs for batched_reduce_size
  Fixes checkstyle errors
  Allow scripted metric agg to access `_score` (elastic#24295)
  [Test] Add unit tests for HDR/TDigest PercentilesAggregators (elastic#24245)
  Fix FieldCaps documentation
  Upgrade to JUnit 4.12 (elastic#23877)
  Set available processors for Netty
  Painless: Fix method references to ctor with the new LambdaBootstrap and cleanup code (elastic#24406)
  Doc test: use propery regex for file size
  [DOCS] Tweak doc test to sync_flush
  ...