Ledger state query for number of stake pool delegations

Introduce a ledger state query that answers how many total delegators to stake pools there is:
```haskell
data WhichSnapShot
= CurrentState
-- ^ Not from any particular SnapShot, but form current Ledger State instead
| MarkSnapShot
| SetSnapShot
| GoSnapShot

queryNumStakePoolDelegators ::
NewEpochState era ->
WhichSnapShot ->
Set (KeyHash StakePool) ->
-- ^ Filter which stake pools this query should return the count for. Empty argument will include all registered stake pools
Int
```
This query will use `numDelegators` field instead of iterating over accounts