Ensure that there are no race conditions in REST API /query endpoint

[bogdankostic]

The REST API can run multiple request concurrently by setting CONCURRENT_REQUEST_PER_WORKER. For concurrent requests, it’s important that we don’t have any race conditions inside the application to ensure the responses are correct. Let’s check our REST API thoroughly for race conditions.

[mayankjobanputra]

1. Uvicorn has worker settings for parallel requests; use it and get rid of our concurrency_limiter
2. Talk about REST API unit tests with Massi

[masci]

We plan to rewrite the rest api and move to async pipeline execution, let's not invest time here