Apply new asyncio syntax and API

[erikyao]

Python 3.10 asyncio syntax and API should be applied across the myvariant.info code base. Specifically:

1. delete all @asyncio.coroutine annotations; use async def to define a corotine
2. use await instead of yield from to consume a corotine
3. consider using asyncio.create_task() instead of asyncio.ensure_future() to schedule a task (which wraps a corotine).

Note that Goal 3 could be tricky because if a corotine is to be executed in another thread (e.g. the corotine JobManager.defer_to_thread()), asyncio.run_coroutine_threadsafe() should be used. See https://docs.python.org/3/library/asyncio-dev.html#concurrency-and-multithreading

Fixes will be pushed to branch py3.10_asyncio_pymongo4

[erikyao]

Fixed with PR https://github.com/biothings/myvariant.info/pull/157