Support async iterator in ResultProxy

[asvetlov]

async for row in conn.exectute("SELECT ..."):
    print(row)

[jettify]

I tried to implement and found that this API is not very good, because there is no way to close created ResultProxy object with cursor inside. We should encourage:

async with conn.execute("SELECT ...") as result:
    async for record in result:
        print(record)

[asvetlov]

Maybe. I used dirty trick to close result IIRC.

[jettify]

resolved by #94