tpcc: consider adding deferred execution mode to delivery

[jordanlewis]

Currently, the delivery transaction is executed synchronously by the worker that picks it. This runs counter to the spec, which states that

The Delivery transaction must be executed in deferred mode. This mode of execution is primarily characterized by queuing the transaction for deferred execution, returning control to the originating terminal independently from the completion of the transaction, and recording execution information into a result file.

However, other load generators like the Percona TPCC implementation don't implement this either. Do we need to?