Rotating the token's bound key

[jricher]

§6.1 Rotating the Access Token: Editor's note:

If the client is using its own key as the proof, like with a bearer access token, the AS is going to need to know if the client's key has been rotated. We don't have a mechanism for rotating the token's key or the client's key yet either - so that could occur through this management function as well.

[jricher]

See discussion at #105

[jricher]

Token key rotation is covered by #435, and a non-bound token would use whatever's currently tied to the client at the AS, whatever that is.