feat(validators): trickle validators on Layer 1

[ksrichard]

Context

Validators are now controlled by Layer 2, so it decides which validator will be active and when.
This responsibility should be transferred to Layer 1, so we can have proofs that what is the real active validator set (Merkle root hash of them).
Layer 2 should only call Layer 1 through gRPC periodically (or when there is a new epoch) whether what are the currently active validator nodes.

Proposal

Similar logic should be applied on Layer 1 that we have now on Layer 2, so when a new validator node is registered (a new L1 transaction is made), then the new validator node is registered with a different start/end epoch if there are enough validators in the next epoch (so as in L2 just look for the next available epoch).
Meanwhile Layer 2 node can poll Layer 1 (at the start of each epoch) to fetch any new VNs and if there are any, and save them in it's own local DB.