async: switch to uvloop

[oliver-sanders]

uvloop is a fast (C) replacement for the built-in asyncio event loop (Python) that uses the libuv event loop.

Basically it makes asyncio go faster so should make async code more responsive. As the Cylc codebase becomes increasingly async this switch will help Cylc efficiency.

Two line change (+ one new dependency).

See also cylc/cylc-flow#4852

The results of profiling this problematic workflow revealed that the asyncio.ensure_future method was consuming a reasonable amount of CPU. It's likely that uvloop will have lower overheads for these operations.

[oliver-sanders]

Turns out the large number of async tasks was the result of a recursive async routine.

#548 turns this into an iterative routine bypassing the issue.