For situations where no credentials are needed but where a working
CredentialsSource is still required, this variable provides a convenient
way to get a fully-functional-but-empty credentials source.
This credentials source wraps another and caches its results in memory
in a map that is keyed on the request hostname.
This should ease the common case of many services operating on the same
hostname by allowing an initial set of credentials to be re-used for
subsequent requests.
Martin Atkins