Transactions needs access to the durable_kv_timeout

When constructing a transactions object, we have only the cluster object.  So we have no way of knowing the durable_kv_timeout being used by the cluster, which should be our default kv_timeout for transactions, and communicated to the query server when doing transactional queries.   

Perhaps an accessor to get the private _origin would be sufficient - you can get whatever you want from that.  This blocks TXNCXX-248