Use ModifiedIndex and not EtcdIndex for watches

When watch returns, it's ModifiedIndex that contains
the index of the returned event. So its incremented value
should be used on next watch.

Fixes #48

subnet/subnet.go

@@ -359,7 +359,7 @@ func (sm *SubnetManager) WatchLeases(receiver chan EventBatch, cancel chan bool)
 }
 
 func (sm *SubnetManager) parseSubnetWatchResponse(resp *etcd.Response) (batch *EventBatch, err error) {
-	sm.lastIndex = resp.EtcdIndex
+	sm.lastIndex = resp.Node.ModifiedIndex
 
 	sn, err := parseSubnetKey(resp.Node.Key)
 	if err != nil {