More cache unification

This is yet another **refactoring** PR.

The caches for ```StarSystem``` and ```Sector``` had a strange asymmetry without a good reason. This was creating ```RefCountedPtr``` cycles in my next refactoring work, which is getting rid of the global ```Galaxy``` object. So, I decided to clean that up first (it was a part of the ```no_global_galaxy``` work, but the latter is already quite large, so I decided to put this one separately).