Currently we iterate over all of the accounts on the epoch boundary, in order to remove delegations whenever stake pools are being retired.
This is very inefficient and is a blocker for LegderHD.
We need to:

• Create StakePoolState: Create StakePoolState #4989
• Add a Set with staking credentials that delegate to the pool
• Maintain the invariant of the above Set being updated whenever stake credentials unregister or redelegate
• clear out all delegations when stake pool retires in a more efficient manner.

This is currently blocked on #4989 and #4694