Aggregations: Track scores should be applied properly for `top_hits` aggregation.

Closes #6934
[TEST] top_hits aggs with breath_first collect mode only fails if scoring is used (score based sort or track_scores)
Docs: Update documentation about execution hints for the terms aggregation.
[Engine] `index.fail_on_corruption` is not updateable

The `index.fail_on_corruption` was not updateable via the index settings
API. This commit also fixed the setting prefix to be consistent with other
setting on the engine. Yet, this feature is unreleased so this won't break anything.

Closes #6941
[RESTORE] Fail restore if snapshot is corrupted

today if a snapshot is corrupted the restore operation
never terminates. Yet, if the snapshot is corrupted there
is no way to restore it anyway. If such a snapshot is restored
today the only way to cancle it is to delete the entire index which
might cause dataloss. This commit also fixes an issue in InternalEngine
where a deadlock can occur if a corruption is detected during flush
since the InternalEngine#snapshotIndex aqcuires a topLevel read lock
which prevents closing the engine.

Closes #6938
[TEST] Activate test in PluginManagerTests that is supposed to be fixed
[TEST] Stress test for update and delete concurrency.

This test deletes and updates using upserts documents over several threads in a
tight loop. It counts the number of responses and verifies that the versions at
the end are correct.
Merge branch 'master' of github.com:elasticsearch/elasticsearch
[TEST] Plugin Manager

Undo removal of change.
Merge branch 'master' of github.com:elasticsearch/elasticsearch
[TEST] Remove TransportUpdateActionTest

This test has been made obselete by the UpdateTests.
Docs: Typo
typo

causes the example to fail in bash
[TEST] Stress Update/Delete

Create the test index with merging disabled so deletes don't get merged away
this was breaking the version calculation.
More lenient type parsing in histo/cardinality aggs
closes #6948
closes #6893
[TEST] Stress update/delete

Reduce top end of scaling parameters to avoid OOMs on nightly runs.
[Discovery] remove unneeded cluster state serialization during cluster join process

At the moment we serialize the cluster state in JoinResponse and ValidateJoinRequest. However this state is not used anywhere and can be removed to save on network overhead

Closes #6949
Test: Set number_of_replicas to zero so that wait_for_green succeeds in search.aggregations
[Discovery] fix MembershipAction.sendJoinRequestBlocking should now expect an empty response

The change introduced in #6949 (do not serialize the cluster state) also means master now responds with an empty response rather then a JoinResponse. However, sendJoinRequestBlocking still expected a JoinRequest.
IndicesStore fix logging format