lib: split nb_operation into two types

lib: split nb_operation into two types

Currently, nb_operation enum means two different things - edit operation
type (frontend part), and callback type (backend part). These types
overlap, but they are not identical. We need to add more operation
types to support NETCONF/RESTCONF integration, so it's better to have
separate enums to identify different entities.

Signed-off-by: Igor Ryzhov <iryzhov@nfware.com>