Enhance /datasets/data-status and /uploads/data-status endpoints

[yuanzhou]

The following enhancements to the /datasets/data-status and /uploads/data-status endpoints are needed in order to proceed with x-atlas-consortia/data-ingest-board#250:

• Do not spawn new jobs when there's a running job
• Response with 202 status if caching is in progress

Understand and port over the necessary implementation from SenNet: https://github.com/sennetconsortium/ingest-api/pull/625/files

According to @tjmadonna:

We refactored the caching from APscheduler to RQ. The only change to the data-status endpoints is that it will return a 202 if the scheduler is currently caching and there is no cache available. The same functionality can probably be done with APScheduler and a couple of flags in Redis.

The dashboard is so we can monitor jobs like entity/metadata validation and registration from users. I've been working on some changes that allows us to see the data-status caching jobs too. This is related to RQ so it shouldn't affect you.