Idea: Let's get rid of the UpdateRegistration RPCs entirely.

According to RFC8555, there are only 3 ways that a client can initiate a change to their Account object:

• POSTing to the Account endpoint with status: Deactivated (this deactivates the account)
• POSTing to the Account endpoint with different Contacts (this updates the contacts)
• POSTing to the separate KeyChange endpoint (this updates the Key)

The first and second of these use the same client-facing endpoint.
But in our codebase, the second and third of these use the same RA and SA codepath (UpdateRegistration).

We should instead have three wholly separate codepaths, so that we don't have to deal with generic functions like mergeUpdate which make it likely that we accidentally allow clients to modify accounts in ways they shouldn't be able to.