Optimise for loop of MigrateToCold

[satushh]

What type of PR is this?

Other

What does this PR do? Why is it needed?

The for loop in MigrateToCold function was brute force in nature. It could be improved by just directly jumping by slotsPerArchivedPoint rather than going over every single slot.

for slot := oldFSlot; slot < fSlot; slot++ {
  ...
   if slot%s.slotsPerArchivedPoint == 0 && slot != 0 {

No need to do the modulo for every single slot.
We could just find the correct starting point and jump by slotsPerArchivedPoint at a time.

Which issues(s) does this PR fix?

Fixes #

Other notes for review

Acknowledgements

• I have read CONTRIBUTING.md.
• I have included a uniquely named changelog fragment file.
• I have added a description to this PR with sufficient context for reviewers to understand this PR.