Add Elasticsearch 7 support, prepare 7.0.0 release

Before submitting the PR make sure the following are checked:

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

toptal / chewy