Add NONE option to conflict detection level

Many HBase use cases are for write-once/append-only data. In that case transaction support is still useful, as it can be used to keep secondary indexes transactional consistent with the table data. However, for this type of data, there's no need to do conflict detection. It would be useful to still use the TransactionAwareHTable as it handles setting the Cell timestamps and tracking what needs to be undone when a transaction should be aborted.