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. 
And yeah I think it would be fine to make those other things public, but I don't think it matters that much.