Additional Nonce Validation

Nonce values are tokens that are stored in the database that prevent
replay attacks, for example if a POST from an administrative user is
intercepted and re-posted by a malicious 3rd party. 

If a Nonce timestamp is older than 5 minutes, refuse the request. 

If a Nonce value already exists in the database, refuse the request. 

If a Nonce is missing user_id, resource_id, or roles parameters, refuse
the request.

Clean our database of existing Nonces once every hour.