Offline: Tie service or request to existing client

[danielfe]

When online requests are always associated with a client and services are sometimes, usually an existing client. But offline we only allow adding new clients because we don't yet have a secure way to keep the client list offline. This issue tracks the ability to store the list of clients offline for use when creating a request or service online.

Blocked on offline authentication #559. Also requires offline encryption of anything stored. Also needs research/design to decide if CBOs should opt in to this or be able to choose identifiers that are stored offline before names, or otherwise take additional security precautions.