Rewrite OdkCentral.py to be async

[spwoodcock]

Is your feature request related to a problem? Please describe.

• Most of the OdkCentral code is network IO bound.
• Using async code is much more efficient for this use case, as code is executed in an event loop: while waiting for the response from some IO operation, another process can be executed (essentially in parallel), instead of locking the thread waiting for the response.
• All modern web APIs in Python are ASGI now, so the default is async upstream. Usage of a sync library in an async framework is problematic.

Describe the solution you'd like

• Update requests --> aiohttp or httpx.
• Rewrite all class methods + functions to be async:
  • Add async def everywhere.
  • await database calls.
  • await network operations.

Describe alternatives you've considered

• Keeping everything as synchronous code, as it's a lot of work to update all of our dependent libraries.

Additional considerations

• This should wait until a good point when we have time to refactor lots of code.