Important API refactoring

- Rename norns_register_backend to norns_register_namespace
- Rename norns_unregister_backend to norns_unregister_namespace
- Rename norns_update_backend to norns_update_namespace
- API is now more symmetrical: all norns_*_namespace functions
receive an nsid as first parameter (in previous versions this
was not consistent).
- The norns_job_t descriptor now requires a norns_job_limit_t
parameter that defines what the job will be allowed to do, rather
than a norns_backend_t specification as in previous versions.
- Other changes: message passing logic improved