PoS Staking Manager should count how many delegators each validator has

[geoff-vball]

Context and scope
If a validation ends before all of it's delegations, we keep a number of objects around in state such as uptime so that the delegators have enough information to exit. We should keep a count of the delegators that need this information so that we can clean it up upon exit.

Discussion and alternatives
We may just want to keep an array of active delegation IDs for each validator. Currently I don't think it's possible for a validator to get any insight into its delegators just by querying the contract state.

Open questions