Consider replacing Tornado with modern async server/framework

[stefanv]

Motivation

• Tornado is, nowadays, slower than some other frameworks.
• We built numerous features ourselves that are, now, supported by frameworks; e.g., API docs & validation.

Options

• FastAPI / starlette
• aiohttp
• uvicorn

aiohttp and uvicorn are fast, but both low level. FastAPI + starlette gives us approximate parity with what we have in our customized Tornado setup.

Strategy

Implement this on a branch first, to allow SkyPortal to try it out. This may introduce a significant amount of work for that project.