add ThriftHiveMetastore.get_open_txns(long txnid)

we currently have ThriftHiveMetastore.get_open_txns() which maps to TxnHandler.getOpenTxns(). The usual usage is TxnUtils.createValidReadTxnList(GetOpenTxnsResponse txns, long currentTxn) where the complete list transactions is obtained from Metastore and then anything above currentTxn is thrown away.
Would be useful to add ThriftHiveMetastore.get_open_txns(long txnid) and TxnHandler.getOpenTxns(long) to not retrieve things that will be thrown away. Especially when there are a lot of running transactions.