set current epoch based on hotshot block height

[alysiahuggins]

The StakeTable smart contract needs to be aware of epochs (which are managed in the consensus layer).
In HotShot, 1 epoch = x hotshot blocks. Since the Light Client contract gets the latest finalized state from espresso once every 3-6 hours, we'll use that state to get the hotshotblock and then calculate the epoch in the Stake Table smart contract.

Assumptions

• the hotshotblocks per epoch doesn't change (and when/if it changes we'll upgrade the contract)
• the light client contract will be maintained and receive the latest state from espresso at least once a day