raft/node.go
func (rd Ready) appliedCursor() uint64 {
	if n := len(rd.CommittedEntries); n > 0 {
		return rd.CommittedEntries[n-1].Index
	}
	if index := rd.Snapshot.Metadata.Index; index > 0 {
		return index
	}
	return 0
}
func (rd Ready) appliedCursor() uint64 {
	if n := len(rd.CommittedEntries); n > 0 {
		return rd.CommittedEntries[n-1].Index
	}
	if index := rd.Snapshot.Metadata.Index; index > 0 {
		return index
	}
	return 0
}
// appliedCursor extracts from the Ready the highest index the client has
// applied (once the Ready is confirmed via Advance). If no information is
// contained in the Ready, returns zero.