Prevent redundant discovery requests

[chlowell]

Public and confidential clients send redundant discovery requests because silent and non-silent (audible? 😛) authentication request and cache discovery metadata independently. For example, AcquireTokenOnBehalfOf
first attempts silent authentication, which requests instance metadata and caches it with storage.Manager. When silent auth fails, it then requests a token from the STS via oauth.Client, which requests instance and tenant metadata and caches it on an instance of oauth.authorityEndpoint.