Fix trio with multiple workers

[miracle2k]

The shutdown_trigger argument of worker_serve needs to be a callable returning an awaitable, not an awaitable directly.

Without this, I get:

  File "/Users/michael/Library/Caches/pypoetry/virtualenvs/dbserver-k0bMYWkt-py3.7/lib/python3.7/site-packages/trio/_core/_run.py", line 730, in __aexit__
    raise combined_error_from_nursery
  File "/Users/michael/Library/Caches/pypoetry/virtualenvs/dbserver-k0bMYWkt-py3.7/lib/python3.7/site-packages/trio/_core/_run.py", line 730, in __aexit__
    raise combined_error_from_nursery
  File "/Users/michael/Library/Caches/pypoetry/virtualenvs/dbserver-k0bMYWkt-py3.7/lib/python3.7/site-packages/hypercorn/utils.py", line 156, in raise_shutdown
    await shutdown_event()
  File "/Users/michael/Library/Caches/pypoetry/virtualenvs/dbserver-k0bMYWkt-py3.7/lib/python3.7/site-packages/hypercorn/utils.py", line 156, in raise_shutdown
    await shutdown_event()

Whenever multiple workers are used.

[pgjones]

Thanks @miracle2k - I'm going to try solve another bug (#24) before I release.