Make State load only from scratch and not into existing state

[tertsdiepraam]

This allows us to get rid of some tracking of whether policies and zones are added, retained, removed or updated.

[ximon18]

What prompted this change? Is this functionality okay to remove? Is the removed behaviour something we never used or needed?

[tertsdiepraam]

I believe that we will never read state after starting up (maybe until we get to high availability but at that point we probably need to reconsider anyway).

[bal-e]

Awesome! Simplifying this makes it easier for us to control the zone initialization code (which would be useful for resolving some workarounds in #552, although we'll get to them later).