Manages an internal list of secret keys used to sign new authentication tokens as they are
generated, and to valid existing tokens used for authentication.
A single instance of AuthenticationTokenSecretManager will be running as the "leader" in
a given HBase cluster. The leader is responsible for periodically generating new secret keys,
which are then distributed to followers via ZooKeeper, and for expiring previously used secret
keys that are no longer needed (as any tokens using them have expired).