Merge branch '279-create-client-state' into 'master'

Create local state for clients at startup. #279

Most of the clients we've written so far use private groups shared with individual contacts and/or a local group that's not shared with anyone. To make it easier to ensure that the necessary groups exist when we need them, this patch allows clients to register startup hooks for creating their local state.

Fixes #279.

See merge request !131