The alive nodes is under threshold, consensus should be stopped

At the booting state, node checks the validators are alive or not, and then start to consensus. During consensus, if some validators are dead or does not respond and the number of alive validators are under threshold, what is the proper action of node?

Currently if under threshold, node keep producing `EXPIRED` ballot, because of timeout. I think, node should be back to the `node.StateBOOTING` or the other new state instead of `node.StateCONSENSUS`.