Add individual user overrides to the DKIM Registry contract

[Divide-By-0]

One way for users to remove their dependence on the public key registry would be to only allow future keys to change once they have approved that. Spec changes:

• add the current block timestamp to the set functions to the dkim key registry contract
• add the userOverrideEnabled mapping of user -> timestamp, and userOverrideKeys mapping of (user, domain) -> key
• add the user's account id (i.e. salted + hashed email) as a parameter on get queries for the registry contract
  • if the userOverrideEnabled mapping contains that user, then only return the key value if 1) the timestamp on the field it queries is before the user's set timestamp or 2) if the timestamp is after, then return the userOverrideKeys value of (user, domain) -> key
• have a function that lets that user's account id change their approval public key via an ether-email-auth proof
• add a function to update userOverrideKeys mapping that lets some EOA update the value of that key to a different value
• add a function for updateApprovalTimestamp that lets the user change the timestamp on userOverrideEnabled to the current timestamp