Include recently slashed churn in exit churn queue

[JustinDrake]

Addresses #527 in combination with #784.

These changes were made to keep a tighter bound on validator churn to ensure adequate safety margins when changing v-sets

• add the recently slashed balances to the exit churn to prevent too much turn over in the case of slashed exits
• do not process exits at all if too long since last validator registry change to avoid counting errors wrt slashed balance array

[vbuterin]

👍

[djrtwo]

Why are you blocking exits if there have been LATEST_SLASHED_EXIT_LENGTH epochs since last registry update?

[JustinDrake]

See discussing here. It's a simplification to avoid caching a slashed_balance.