fix: Uvicorn graceful shutdown.

[DABND19]

For this example graceful shutdown for uvicorn service doesn't work:

import asyncio

import aiomisc
import fastapi
from aiomisc.service.uvicorn import UvicornService

app = fastapi.FastAPI()

@app.get('/hello')
async def hello_world():
    await asyncio.sleep(15)
    return {'message': 'Hello, world!!!'}


class API(UvicornService):
    async def create_application(self) -> fastapi.FastAPI:
        return app


if __name__ == '__main__':
    with aiomisc.entrypoint(API()) as loop:
        loop.run_forever()