racing risk in MultiTxn's serialize

[happen]

Hi


     I think may be there is a little risk when running 
MultiTxn's serialize txns.get(vidx1) step. Because probably the txns' size is 
changing that time in the future.


```
public void serialize(OutputArchive a_, String tag) throws java.io.IOException {
  a_.startRecord(this,tag);
  {
    a_.startVector(txns,"txns");
    if (txns!= null) {          int len1 = 
txns.size();
&nbsp; &nbsp; &nbsp; &nbsp; for(int vidx1 = 0; vidx1<len1; vidx1++) {
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; Txn e1 = (Txn) txns.get(vidx1);
&nbsp; a_.writeRecord(e1,"e1");
&nbsp; &nbsp; &nbsp; &nbsp; }
&nbsp; &nbsp; }
&nbsp; &nbsp; a_.endVector(txns,"txns");
&nbsp; }
&nbsp; a_.endRecord(this,tag);
}
```