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. 
Oh. I didn't notice the default was true. Hmm..

It seems pretty magical to me. I think I would prefer to see something like ignore_kwargs=['log'] at call-sites, and not defaulting to anything.