We currently call `onSlotEnd` whenever all in-BN validator duties are
completed. VC validator duties are not awaited. When `onSlotEnd` is
processed close to the slot start, a VC may therefore miss duties.
Adding a delay before `onSlotEnd` improves this situation.
The logic can be optimized further if `ActionTracker` would track
`knownValidators` from REST separately from in-process ones.