* Initial implemntation of returning 529 for rate limits
- bump aws iam and sts packages to v1.14.31 to get mocking interface
- promote the iam and sts clients to the aws backend struct, for mocking in tests
- this also promotes some functions to methods on the Backend struct, so
that we can use the injected client
Generating creds requires reading config/root for credentials to contact
IAM. Here we make pathConfigRoot a method on aws/backend so we can clear
the clients on successful update of config/root path. Adds a mutex to
safely clear the clients
* refactor locking and unlocking into methods on *backend
* refactor/simply the locking
* check client after grabbing lock
