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
Removed Index Status API

The functionality of the index status API has been replaced by the recovery API.

Relates #4854