Add Stake Table for first 2 epochs

[ss-es]

HotShot will need a stake table for the first few epochs, which should be the continuation of the current stake table. However, the Membership implementation has no way of knowing which epoch number will be the "first" epoch.

We should add a new method to Membership that looks something like set_first_epoch(&mut self, epoch: TYPES::Epoch) -> Result<()>, which HotShot will use to mark the first "real" epoch number by calling set_first_epoch on the current consensus epoch number immediately after the upgrade is decided.

The Membership implementation should insert its initial non-epoch stake table at both the given and next TWO epochs (for a total of 3 epochs). In most cases only 2 will be needed, but in an edge case where the upgrade is decided towards the end of an epoch we may not call add_epoch_root on it.

Since nodes may not decide upgrades at the same point, we should ensure (when deploying) that the contract provides the same stake table as the initial stake table for all 3 of these epochs and is not modified until after the 3rd real epoch.