Turn DMTN-056 into a polished, internally-consistent proposal for a new Butler design.

This should include:

• Python and SQL interfaces at and above the Registry and Datastore level
• walkthroughs of how the design supports various use cases
• a preliminary list of the concrete Registry and Datastore implementations needed.

This is the ticket for Jim Bosch's piece of the work; a similar ticket will follow for Pim Schellart [X].