Stores a "metadata" string from the caller in Redis, keyed by a hash of
the message id. In practice the metadata is JSON in the auth server's
case, but this repo neither knows or cares about that. No functionality
is added here to read or clear the data from Redis, that's coming in a
separate changeset for the queues process.
The HMAC key for hashing the message id comes from config, obviously,
because it's secret. It has a very specific name at the moment, but we
should feel free to rename it to something more generic than that if we
have other data that we'd like to hash with it in the future.