Replace parent-child mapping with a join field

[mrzasa]

ES 7.0 support: https://www.elastic.co/guide/en/elasticsearch/reference/current/removal-of-types.html#parent-child-mapping-types

Remove parent/child mapping with parent param, replace with join field.

---

Before submitting the PR make sure the following are checked:

• [ ] The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• [ ] Wrote good commit messages.
• [ ] Commit message starts with [Fix #issue-number] (if the related issue exists).
• [ ] Feature branch is up-to-date with master (if not - rebase it).
• [ ] Squashed related commits together.
• [ ] Added tests.
• [ ] Added an entry to the changelog if the new code introduces user-observable changes. See changelog entry format for details.