chore(state): Save validator set in shim layer state

[jmalicevic]

Currently, Emerald always asks the smart contract for the current validator set. Ideally we should have this state in our local DB to avoid calls to fetch the valset.

These entries would be added to the database on start of the node and when the validator set changes.
Possible we'd have to still call into the execution engine on Decided but we will save calls in other places.

[bastienfaivre]

In addition to what has already been done in #142, let me know if I understand correctly what still needs to be implemented.
The idea is to store the validator set in the local DB and update it whenever it changes. Then, on startup, we check if we already have the validator set in the DB before trying to call the smart contract.
Moreover, are the validator set updates purely dependent on the smart contract events ValidatorRegistered, ValidatorUnregistered, ValidatorPowerUpdated? If so, we could update (call the contract), only when such events occur, and not after each Decided event.
However, the code is not listening to these events yet. And I think it would be way easier to keep updating the validator set after each Decided event for now.