asgi: import app and start app in same event loop

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hypercorn import package and run asgi in different event loop. make it hard to call asyncio.get_event_loop() to get event loop on module init, and use it in asgi application.

For example, this works fine with uvicorn:

pg = asyncpg.create_pool(dsn=PG_DSN)

# asgi lifespan
async def startup():
    await pg

this will cause RuntimeError: Task <Task pending name='Task-2' coro=<Lifespan.handle_lifespan() running at ...\hypercorn\asyncio\lifespan.py:55>> got Future <_OverlappedFuture pending overlapped=<pending, 0x17c193fcf70>> attached to a different loop