Store and Load Epoch information

[bfish713]

When we startup we need to load information about our last current known epochs. This can be stored and loaded in the Membership directly or passed to the HotshotInitializer

Calls to add_epoch_root should effectively be stored in persistence. When we start back up we'll need to fetch

• The current stake table for the epoch OR the L1 height for the current epoch so we can just sync with L1
• The epoch roots and/or stake tables for any future epochs we got a root for (could be the next 2 epochs
• The DRB seed for any of those epochs roots

We should also be sure to load at 2 consecutive epochs for catchup. We need this because odd and even epochs are linked. So if we start with just epoch 11, we can only get the stake table root for epoch 12 (decided in epoch 10) by catching up completely. This is true for any future even epoch (if we are trying to catchup to 20 we need the root decided in epoch 18 which needs epoch 16 stake table to verify, which means we need epoch 16's root etc.)