Wait decorator to disallow re-entrantcy

Implemented `wait` decorator that waits for result when the function is called again without first result being available. Wrapping `_auth_me` to get auth token only once. This was required when `ImpersonatingAuthenticator.authenticate_tenant` is called many times when it is not already having a token. Using it in `CachingAuthenticator` to extract out the waiting pattern. 
I think you won't need this if you use characteristic to define this class, and one of my outstanding branches does this: PR #794.