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.

fix delete_nodes and more consistent naming