Detect validator set changes

[ethanfrey]

For the validator sync protocol, we need to send messages when a new validator enters the active set for the first time (can be repeated everytime they enter), and to remove them when tombstoned (not when temporarily inactive). For this, we need some custom SDK logic that would call into the contracts. It must be transmitted by converter, but the callback can go to virtual-staking, which can take care of relaying it, as that is the one point of integration with the sdk