Refactor use of Session structure to store all variables in a service-specific nested structure.

[pheller]

user_id is set by the Logon service and should be stored within a nested Logon structure within Session; similarly, the Messaging related Session data should be stored in a nested Messaging struct.

Will be necessary to update all dependencies that make use of the Session.

[pheller]

See #97 for how messaging uses the session struct