Typespec a bit of escalus_users
Refactor escalus_users to realize escalus_user_db behaviour
Resolve conflict: use escalus_user_db instead of escalus_auth

escalus_auth was used both as a global option for specifying user
authentication method and as a user creation/deletion method description.

User creation/deletion (in general, user server side management) is a different
concept than client authentication, so make the names distinct.