New Redis integration #3018 #3100

[washerath]

---

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?

[FlorianHockmann]

Thanks a lot for contributing this, @washerath! Before diving too deep into reviewing the changes, I have some high-level questions:

1. Would it make sense to move this into an independent module like janusgraph-redis? That way, we wouldn't have to let janusgraph-core depend on a Redis library and I think it would also make it easier to maybe add other cache implementations in the future.
2. Are you already using this implementation yourself? Maybe even in production? Could you share some experiences with it? (This is of course not necessary. I'm just curios here.)
3. @li-boxuan asked in your other PR about performance. Do you have anything to share about that?

I also think that we need some documentation for this. Otherwise users won't even know that it exists. Maybe we should even add a new section "Cache Backends" similar to our "Storage Backends" and "Index Backends".
But we can also do that in a follow-up PR I think.