[DOCS] clarification of breaking changes to 1.4 due to GET index API
[TEST] Use a real unique clustername for InternalTestClusterTests
Spec: Fixed the docs URL for indices.get and indices.exists
Changed dev-tools/es_release_notes to:
* remove markdown support
* prefer PRs over issues
* include related issues in the list
* always use section headers if there is a header
[STORE] Improve exception from Store.failIfCorrupted

If you have previously corrupted files, this method currently builds an
exception like:
```
    failed engine [corrupted preexisting index]
    failed to start shard
```

Followed by a CorruptIndexException. This commit writes the entire
stacktrace to provide additional information. It also changes the
failure message from `corrupted preexisting index` to `preexisting
corrupted index` to prevent confusion.

Closes #7596
Aggregations top_hits: Fixed inconsistent sorting of the hits

In the reduce logic of the top_hits aggregation if the first shard result to process contained has no results then the merging of all the shard results can go wrong resulting in an incorrect sorted hits.
This bug can only manifest with a sort other than score.

Closes #7697
More Like This API: remove unused search_query_hint parameter

Closes #7691
[TEST] Use a sorted set since sets are compared and compare is order specific
[TEST] fix another rounding issue
[STORE] Turn unexpected exception into CorruptedIndexException

Today if we run into exception like NumberFormatException or IAE
when we try to open a commit point to retrieve checksums and calculate
store metadata we just bubble them up. Yet, those are very likely index
corruptions. In such a case we should really mark the shard as
corrupted.