Specify a rendezvous service for consumer reconciliation

[amrc-benmorrow]

This needs to be able to track requests made by consumers and fulfillments coming from the reconciliation service. This means it needs stable storage (this is another database). We also need Sparkplug notification; at minimum change-notify, though it would be good to have enough information available that at least some activities didn't need to hit the REST API.

It's possible this function could be filled by the ConfigDB, but getting the permissions right is currently impossible. It's also difficult to see how to handle multiple sessions for the same user. If we keep all of a user's requests under their own UUID we then need to handle concurrent updates; we have If-Match, but this still relies heavily on each application being careful to only modify its own state. If we mint a new UUID for each application session this again become difficult to grant permissions for and ends up with an awful lot of UUIDs registered.