rename Connection.wait_closed()

[asvetlov]

asyncio.AbstractServer uses the following idiom:

• close() closes connection asynchronously, it's regular function.
• wait_closed() is a coroutine that waits for actual closing.

Connection.wait_closed() has different behavior: it sends disconnection signal to server and after that closes connection.

I guess rename it to .ensure_closed().

[jettify]

+1. I missed idea behind wait_closed()