Followup work from WT-3142. We should be able to run .wtperf files directly as workgen workloads. With little extra trouble, we should emit the intermediate Python program to do this, as it this will be a helpful tool to create/extend tests.

Our goal is not to run every possible .wtperf file, but rather to run the most common features. We should start with a subset of .wtperf files that are used in Jenkins runs, and once we can do accurate emulations, set up Jenkins runs for the workgen versions. Then we can run them side by side with the wtperf versions to verify they perform as expected.