Most of our field types have the same implementation for their `existsQuery` method which relies on doc_values if present, otherwise it queries norms if available or uses a term query against the _field_names meta field. This standard implementation is repeated in many different mappers.
There are field types that only query doc_values, because they always have them, and field types that always query _field_names, because they never have norms nor doc_values. We could apply the same standard logic to all of these field types as `MappedFieldType` has the knowledge about what data structures are available.
This commit introduces a standard implementation that does the right thing depending on the data structure that is available. With that only field types that require a different behaviour need to override the existsQuery method.
At the same time, this no longer forces subclasses to override `existsQuery`, which could be forgotten when needed. To address this we introduced a new test method in `MapperTestCase` that verifies the `existsQuery` being generated and its consistency with the available data structures.
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