Store sessionId in high level Client object

[OSCSW]

The sessionId is a numeric NodeId for a node created on the server when a session is created. The children of this node provide diagnostic information for the session on the server, and their NodeIds use the sessionId as part of their string type identifier. sessionIds are unique, which also means the diagnostic NodeId strings are unique, making them hard to access without the sessionId.

The lower level UaClient.create_session() already returns the sessionId as part of the response from the server. However it is simply dropped by the Client.create_session(), and instead could be stored in the Client class.

See sessionId in Table 15 https://reference.opcfoundation.org/Core/Part4/v104/docs/5.6.2.2.

[OSCSW]

The change would be 3 lines:
In Client.__init__:
self.session_id = None
In Client.create_session:
self.session_id = response.SessionId
In Client.close_session:
self.session_id = None

Unfortunately I do not have permissions to push a branch.