* Add AWS Secret Engine Root Credential Rotation
This allows the AWS Secret Engine to rotate its credentials used to
access AWS. This will only work when the AWS Secret Engine has been
provided explicit IAM credentials via the config/root endpoint, and
further, when the IAM credentials provided are the only access key on
the IAM user associated wtih the access key (because AWS allows a
maximum of 2 access keys per user).
Fixes#4385
* Add test for AWS root credential rotation
Also fix a typo in the root credential rotation code
* Add docs for AWS root rotation
* Add locks around reading and writing config/root
And wire the backend up in a bunch of places so the config can get the
lock
* Respond to PR feedback
* Fix casing in error messages
* Fix merge errors
* Fix locking bugs
