* move rebuild_working_set logic to replica.rs, also include states !=
  completed / deleted
* [WIP] assign types to properties in Replica
  - db / operation model is just k/v, but formatted names can be used for
    structure:
    - dependencies: `dependency.<uuid> = ""`
    - annotations: `annotation.<epoch time> = "annotation"`
    - tags: `tags.<tag> = ""`
* abstract server into trait
* add HTTP API
* fix TODO items in replica.rs
* implement snapshot requests
* implement backups
* implement client-side encryption
* design expiration
  - need to be sure that create / delete operations don't get reversed
* cli tools
* move test-only tools somewhere else (helpers in tests/?)