propose/implement a replacement for Flux

DLab uses Flux stores for quite a few components - but [Flux is EOL](https://github.com/facebookarchive/flux) and for a variety of reasons we should replace it.

This issue has two steps: Find and propose a solution, then after approval, implement and document.

First, I suggest reviewing dlab's use of flux and then propose a solution to replace it which will handle our use cases. When thinking about a given solution, please give consideration to ease of use, especially for maintaining it. We need something future volunteers are likely to know or find guidance on, so popular = good in this case. I'll do first review and pass the final yes/no call to Marlon, since his time is more limited than mine right now.

Second, once a solution has been approved, proceed with implementation _and_ documenting it. Documentation is just as important as deploying - we've been remiss on documenting best practices/patterns in the past, and for something this fundamental we definitely need that going forward.

If you have any questions or clarifications please reach out to me (comment here, slack in #developers, whatever works)