Ensure that the sov helper is refreshed

- Caches are good, but remember to expire them.
Make sov a dynamic property

- This... causes funny things with the change ownership method, so for
  now maintain compatibility with that, even though this method is
  probably never used in practice.
Correctly calculate new fuel rates for sov changes

- Strontium deltas were not recalculated correctly as it is relatively
  static.
- Updated the test helpers to simulate the refresh, to ensure sov is
  always fresh.
- Adjusted comments, such as removing misleading/misunderstandings.