CEP 15: (Accord) push waiting for epoch into CommandStore rather than require every command to think about the edge case when the epoch isn't ready

We have a common pattern of scheduling work on a CommandStore, but this work has to double check the epoch exists, if not it has to wait on the fetch and reschedule again… this pattern could likely be pushed to CommandStore if the PreLoadContext includes the epochs required (just like we do for commands).

For things outside of CommandStores, they would have to maintain their current logic, the scope of this work is just tasks that run on CommandStores.