The change adds a new option to the `nested`, `has_parent`, `has_children` and `parent_id` queries: `ignore_unmapped`. If this option is set to false, the `toQuery` method on the QueryBuilder will throw an exception if the type/path specified in the query is unmapped. If the option is set to true, the `toQuery` method on the QueryBuilder will return a MatchNoDocsQuery. The default value is `false`so the queries work how they do today (throwing an exception on unmapped paths/types)
* Remove remaining 1.x bwc logic.
* Stop storing stored fields and indexed terms. The _parent field's only purpose is to support joins between parent and child type and only storing doc values is sufficient.
* In the mapping the parent field mapper is now known under '{parent}#{child}' key, because this is the field the parent/child join uses too.
* Added new sub fetch phase to lookup that _parent field from doc values field if that is required (before this was fetched from stored _parent field)
* Removed the ability to query directly on `_parent` in the query dsl. Instead the `{parent}#{child}` field should be used. Under the hood a doc values query is used instead of a term query, because only doc values fields are stored now.
* Added a new `parent_id` query to easily query child documents with a specific parent id without having to know what join field to use
* Also in aggregations `_parent` field can't be used any more and `{parent}#{child}` field name should be used instead to aggregate directly on the _parent join field.
Colin Goodheart-Smithe
Clinton Gormley
Jason Tedor
Martijn van Groningen