Consider doing local hmac with local secret before hitting WebKMS hmac

[dlongley]

This would add additional security by further hiding the values sent to an external WebKMS system. Currently, the values sent to that external (and trusted) system are pre-hashed -- but we could instead hmac them so that rainbow tables could not be used on snooped WebKMS traffic to determine what was sent. Note: Considering this a problem that needs solving may rely on an unnecessary level of paranoia.