extend OpenTxnRequest with transaction type

There is a TXN_TYPE field in TXNS table.

There is TxnHandler.TxnType with legal values. It would be useful to TxnType a Thrift, add a new COMPACTION type object and allow setting it in OpenTxnRequest.

Since HIVE-20823 compactor starts a txn and should set this.

Down the road we may want to set READ_ONLY either based on parsing of the query or user input which can make TxnHandler.commitTxn faster.