Service level RobotId proto default value usage

[fnalmeidap]

Currently, RobotId proto messages sent for the robot number 0 does not have the mandatory field RobotId.number because protobuf 3 does not serialize default values (see here).

As explicit typing this field as optional is a semantic mislead but employs desired behavior of field serialization when default values are set.

A better approach should be implemented to deal with this issue so we can have semantic coherence alongside functionality. A possible solution was discussed at #51