Add a dedicated field data type for the _index field mapper.

This makes aggregations work on the _index field, and also allows to remove the
special facet aggregator for the _index field.

Close #5848
Update keyword-tokenizer.asciidoc
Removed Index Status API

The functionality of the index status API has been replaced by the recovery API.

Relates #4854
Renamed IndicesOptions#strict and IndicesOptions#lenient to make it clearer what they actually return, reused methods and introduced new one

Relates to #6059, where two new constants were introduced in IndicesOptions. There were already two constants there though, one of which we could have reused. This commit tries to unify them.
Add `shard_min_doc_count` parameter for significant terms similar to `shard_size`

Significant terms internally maintain a priority queue per shard with a size potentially
lower than the number of terms. This queue uses the score as criterion to determine if
a bucket is kept or not. If many terms with low subsetDF score very high
but the `min_doc_count` is set high, this might result in no terms being
returned because the pq is filled with low frequent terms which are all sorted
out in the end.

This can be avoided by increasing the `shard_size` parameter to a higher value.
However, it is not immediately clear to which value this parameter must be set
because we can not know how many terms with low frequency are scored higher that
the high frequent terms that we are actually interested in.

On the other hand, if there is no routing of docs to shards involved, we can maybe
assume that the documents of classes and also the terms therein are distributed evenly
across shards. In that case it might be easier to not add documents to the pq that have
subsetDF <= `shard_min_doc_count` which can be set to something like
`min_doc_count`/number of shards  because we would assume that even when summing up
the subsetDF across shards `min_doc_count` will not be reached.

closes #5998
closes #6041
Node version sometimes empty in _cat/nodes
closes #5480
[DOCS] Update nodes documentation with all headers

Adds a table with the exhaustive list of all available headers with a brief description (mostly from `org.elasticsearch.rest.action.cat.RestNodesAction`) so that people do not need to go searching for them in the code like I did, or search through `nodes?help`.
Changed the respnose structure of the percentiles aggregation where now all the percentiles are placed under a `values` object (or `values` array in case the `keyed` flag is set to `false`

Closes #5870