Limit the number of bytes that can be allocated to process requests.

This should prevent costly requests from killing the whole cluster.

Close #6050
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
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.