Manages an internal list of secret keys used to sign new authentication
tokens as they are generated, and to valid existing tokens used for
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).