Fixes#579 (which blocks #132).
This changes the SA to use a unique index on the sha256 of a
Registration's JWK's public key data instead of on the full serialized
JSON of the JWK. This corrects multiple problems:
1. MySQL/Mariadb no longer complain about key's being larger than the
largest allowed key size in an index
2. We no longer have to worry about large keys not being seen as unique
3. We no longer have to worry about the JWK's JSON being serialized with its inner keys in different orders and causing incorrectly empty queries or non-unique writes.
This change also hides the details of how Registrations are stored in
the database from the other services outside of SA. This will give us
greater flexibility if we need to move them to another database, or
change their schema, etc.
Also, adds some tests for NoSuchRegistration in the SA.
Richard Barnes