)

Today we prevent nodes from joining when indices exists that are too old.
Yet, the opposite can happen too since lucene / elasticsearch is not forward
compatible when it gets to indices we won't let nodes join the cluster once
there are indices in the clusterstate that are newer than the nodes version.
This prevents forward compatibility issues which we never test against. Yet,
this will not prevent rolling restarts or anything like this since indices
are always created with the minimum node version in the cluster such that an index
can only get the version of the higher nodes once all nodes are upgraded to this version.

* master: (137 commits)
  CONSOLEify the "using scripts" documentation
  Adds tests for cardinality and filter aggregations
  Add Backoff policy to azure repository
  Revert "Adds tests for cardinality and filter aggregations (elastic#23826)"
  Adds tests for cardinality and filter aggregations (elastic#23826)
  mute testDifferentRolesMaintainPathOnRestart
  Replace custom sort field with SortedSetSortField and SortedNumericSortField when possible (elastic#23827)
  Prevent nodes from joining if newer indices exist in the cluster (elastic#23843)
  Synchronized CollapseTopFieldDocs with lucenes relatives (elastic#23854)
  CONSOLEify analysis docs
  Adapted search_shards rest test to work with Perl
  To examine an exception in rest tests, the exception should be caught, not ignored
  Fixed bad YAML in rest tests
  Fix BootstrapForTesting blowup
  Ban Boolean#getBoolean
  Fix language in some docs
  CONSOLEify lang-analyzer docs
  Stricter parsing of remote node attribute
  Fix cross-cluster remote node gateway attributes
  FieldCapabilitiesRequest should implements Replaceable since it accepts index patterns
  ...

* master: (146 commits)
  Introduce single-node discovery
  Await termination after shutting down executors
  Fix initialization issue in ElasticsearchException
  Fix bulk queue size in thread pool docs
  [DOCS] Remove line about eager loading global ordinals
  GceDiscoverTests - remove intitial_state_timeout
  SpecificMasterNodesIT shouldn't use autoMinMasterNodes
  testRestorePersistentSettings doesn't to mess with discovery settings
  testDifferentRolesMaintainPathOnRestart shouldn't use auto managing of min master nodes
  CONSOLEify the "using scripts" documentation
  Adds tests for cardinality and filter aggregations
  Add Backoff policy to azure repository
  Revert "Adds tests for cardinality and filter aggregations (elastic#23826)"
  Adds tests for cardinality and filter aggregations (elastic#23826)
  mute testDifferentRolesMaintainPathOnRestart
  Replace custom sort field with SortedSetSortField and SortedNumericSortField when possible (elastic#23827)
  Prevent nodes from joining if newer indices exist in the cluster (elastic#23843)
  Synchronized CollapseTopFieldDocs with lucenes relatives (elastic#23854)
  CONSOLEify analysis docs
  Adapted search_shards rest test to work with Perl
  ...