refactor: move DecreaseScores method to be inside CDeterministicMNList class

[PastaPastaPasta]

Issue being fixed or feature implemented

It makes more sense for DecreaseScores to be inside of the MNList itself imo

What was done?

Refactored as such

How Has This Been Tested?

Reindexed

I had originally expected some performance improvements due to the removal of GetMN but in my benchmarking I didn't see any noticeable perf changes. I do still think the removal of GetMN and using a shared_ptr the whole time is better as it removes the chance of the master node disappearing from the list (which would have previously thrown, but is now impossible).

Breaking Changes

None

Checklist:

Go over all the following points, and put an x in all the boxes that apply.

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated relevant unit/integration/functional/e2e tests
• I have made corresponding changes to the documentation
• I have assigned this pull request to a milestone (for repository code-owners and collaborators only)

[UdjinM6]

makes sense, a couple tiny suggestions

[ogabrielides]

utACK

[UdjinM6]

👍

ACK