Pending conf change is not handled correctly during campaign

[BusyJay]

After joint consensus is implemented, we add a new conf change type ConfChangeV2. But when counting pending conf change, only the old one is counted:

etcd/raft/raft.go

Lines 1675 to 1683 in 4c6881f

	func numOfPendingConf(ents []pb.Entry) int {
	n := 0
	for i := range ents {
	if ents[i].Type == pb.EntryConfChange {
	n++
	}
	}
	return n
	}

This can lead to two leader at a term problem.

Another problem that has been fixed in raft-rs is that all type of campaign including election timeout and transferring leader should check pending conf change count. But currently etcd/raft only checks pending conf change when campaign is triggered by election timeout. This can also lead to two leader at a term problem. See also tikv/raft-rs#221.

/cc @tbg