Close async generators created by Executor on Golem shutdown

[azawlocki]

Resolves #481

The following scenario leads to a deadlock:

async with Golem(...) as golem:

    completed = golem.execute_tasks(...)
   
    async for task in completed:
        break  
        # or raise exception, or return from function

On shutdown, Golem is waiting for all registered Jobs to be marked as finished. However, a Job created by Executor.submit() (called from execute_tasks()) is marked as finished only after all tasks are retrieved from the associated async generator (here: completed).

To fix this, in this PR we register each async generator created by Executor.submit() in a Golem instance used by the Executor. When the instance shuts down, it calls aclose() on all registered generators before waiting for the registered jobs.