Several changes (intermediate backup):

Proto:
- Minor corrections in context.proto

Common:
- Minor corrections in common.orm.backend
- Improved definition of orm.Model class with get_instances and get_references
- Extended ORM unitary tests with get_instances and get_references

Context:
- Implemented missing methods in ContextClient
- Started to implement new Context data models using ORM.
- ContextModel functional using ORM and gRPC.
- Arranged old code file placing