update registry to use file (or db?)

[turkosaurus]

Currently, registry is added to the locket server on deploy, as an embedded yaml.

• provide endpoint and authentication mechanism (with CRUD) to update registry
• read from embed only on startup, then create a file and read from it
• use mutex for file updates, also implement backup fallback

This will allow inspection and manual updates when services are (re)deployed, and will allow add or update their public keys with the registry, as well as allowing a simple file transfer to update state rather than redeploying the service to update (as is currently the case).