Handle Asyncio Transports Explicitly

Track transport objects so they can be closed explicitly on server shutdown.

Allows tracking of all asyncio transport objects that are created from connecting clients.

Support passwords with SecurityPolicy#None (plain text)

add tests for connection with crypto

fix race condition in subscription

Create SubscriptionItemData before sending request to server. So we can process notification arrive before request return. The downside is data.server_handle is None before returning result is processed.

Added flexibility by adding functions into server

small fixes

Use correct password encryption algorithm in client

Client must analyze SecurityPolicyURI from the endpoint description,
and then encrypt password with respective asymmetric algorithm.