raft: save removed nodes in snapshot

fix #1208 
The removed one should not talk to this node. And this node should not talk to the removed node until it committed that remove log entry. (After it commit that log entry, it also does not need to talk with the removed one)

And any machine after the recovery(if it does not have the new version of removed nodes) cannot be the leader. Since its committed index is the lowest and it does not have any uncommitted index. It cannot win any election.