[CONFIG] Contacts API and code reorganization.

Summary:
 * Move the contacts code into its own directory as it grows, rename
   rolodex->contacts for greater clarity.
 * Provide a minimal ZeroRPC API for starting/stopping contacts sync, and
   adding/updating/getting contact data.

Test Plan: Unit tests added.

Reviewers: spang

Reviewed By: spang

Differential Revision: https://review.inboxapp.com/D38