Add doc values support to _parent field

PR for #6107
maybe it could be more concise by having something like:

```java
private static final String PARENT_PREFIX = ParentFieldMapper.NAME + "#";

public DocValuesFormat getDocValuesFormatForField(String field) {
  FieldMappers indexName = mapperService.indexName(field);
  if (indexName == null && field.startsWith(PARENT_PREFIX)) {
    String type = field.substring(PARENT_PREFIX.length());
    [...]
  }
}
```