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. 
The first "its" here should be "it's", though I'd probably use "it is" just so it reads a bit better :)