replace gen.coroutine usage with `async def`

most functions that use `@gen.coroutine` were upgraded in #3706 #3242 #2934 and #2871 however there's still a few left

notably
https://github.com/dask/distributed/blob/f4c52e9a6b407fda875a733f20fd43d956f252d3/distributed/comm/tcp.py#L330-L334

this `Task was destroyed but it is pending!` warning message is due to `loop.stop()` being called before all the tasks are finished. When using `asyncio.run` all tasks are cancelled and the loop only stops when all the tasks are finished and so this error should no longer happen.

see also https://github.com/dask/distributed/pull/3245

Soft blocked by: Usage of asyncio.run