Callback URL needs to be mandatory

[sjurcak]

Since discovery_reference is no longer required, there is no easy way to find the callback. Given that there are two possible solutions for Discovery, one might fallback to the old Discovery model, and not pass a discovery_reference. In this case, it would be impossible to contact the USS.

[nasajoey]

Is it required with all UTMMessages? Or just for PERIODIC_POS_REP_START messages? Spec already notes it is required in that case.

[nasajoey]

talked with @myspri and made changes. Closing for now... if still needs fixing, reopen with comments.