Improve session objects

[smithsz]

Currently we have 3 session types:

• ClientSession (inherits from requests.Session)
• CookieSession (inherits from ClientSession)
• IAMSession (inherits from ClientSession)

There's a lot of duplicated logic between CookieSession/IAMSession. These should inherit from a base class AuthSession(ClientSession).

client.py is forever calling if self.admin_party: before performing session functions. This logic should move to the Session object. Perhaps implement a new AdminPartySession where login/logout methods are noop functions.

[ricellis]

Won't fix.