Introduce attempt completion event

[igalshilman]

greeter = Service("greeter")

@greeter.handler()
async def greet(ctx: Context, name: str) -> str:

    e = ctx.request().attempt_finished_event

    async def long_running_task():
        while True:
            if e.is_set():
                print("teardown event set", flush=True)
                return "this does not matter anymore"
            await asyncio.sleep(1)

    await ctx.run("hi", long_running_task, args=())
    return f"Hello {name}!"