WIP: implement ElasticSearch search_after

[cdegroc]

WIP pull request

Related to #986 and #1680

Done

• Add methods in ElasticSearchClient/RestElasticSearchClient to create/delete a point in time
• Add methods to use search_after with a point in time instead of scrolls

To do

• Unit tests for RestElasticSearchClient
• Change ElasticMajorVersion to also provide the minor version so we can allow PIT for ES 7.10+
• Check if any change was introduced while refactoring runCommonQuery and query to use a common code

---

Thank you for contributing to JanusGraph!

In order to streamline the review of the contribution we ask you to ensure the following steps have been taken:

For all changes:

• [ ] Is there an issue associated with this PR? Is it referenced in the commit message?
• [ ] Does your PR body contain #xyz where xyz is the issue number you are trying to resolve?
• [ ] Has your PR been rebased against the latest commit within the target branch (typically master)?
• [ ] Is your initial contribution a single, squashed commit?

For code changes:

• [ ] Have you written and/or updated unit tests to verify your changes?
• [ ] If adding new dependencies to the code, are these dependencies licensed in a way that is compatible for inclusion under ASF 2.0?
• [ ] If applicable, have you updated the LICENSE.txt file, including the main LICENSE.txt file in the root of this repository?
• [ ] If applicable, have you updated the NOTICE.txt file, including the main NOTICE.txt file found in the root of this repository?

For documentation related changes:

• [ ] Have you ensured that format looks appropriate for the output in which it is rendered?

[li-boxuan]

Quick question: how is this PR related to #986?

[cdegroc]

Quick question: how is this PR related to #986?

I don't think this PR can solve #986 alone. But search_after was mentioned in #986 as a (potential) step towards resolving the issue.

[li-boxuan]

I don't think this PR can solve #986 alone. But search_after was mentioned in #986 as a (potential) step towards resolving the issue.

Got it. Yeah, now I can see how it's related. I am also working towards #986 but from a different aspect (passing gremlin range step information into index query). I was just asking to ensure there is no overlapping :)