Add a workload generator application

Adding new features to wtperf requires more effort than we would like, in part because parsing the config for complex structures requires custom code, and in part because the code has had various features bolted on over time.

For some time, we have toyed with the idea of a workload generator where Python creates and configures the database, then describes a workload which is handed off to native code to execute. This would separate the concerns of specification from execution.