JSON-RPC: Review {instrument,country,...}Id usage

Some APIs work with serialized data (skill level, directory server status), some other APIs return or expect internal ids such as instrumentId or countryId.

**Has this feature been discussed and generally agreed?**

No.

**Describe the solution you'd like**

We should define how this is supposed to be.
1. Do we try to be user-friendly and serialize everything to a proper string so that the API and the implementation are rather disentangled?
2. Do we keep implementation overhead low and expose internal ids via the API?
3. Do we decide on a case-by-case basis?

As an API-person, I tend towards (1). It's also becoming obvious that making implementation details such as the countryId being a Qt5 QLocale::Country has downsides.

**Describe alternatives that have been considered**

See above.

Start by reviewing the existing JSON-RPC APIs that serialize skill level and directory server status, along with APIs using instrumentId or countryId. Compare the current handling of these values and resolve whether the API should expose strings, internal ids, or case-by-case representations; done requires an agreed consistent specification.

Written by the indexing model from the issue text.