Add ability to cycle through create and drop tables in workgen

With WT-7290 we attempted to import many-dhandle-stress.wtperf to workgen, and realized that workgen doesn't have a wtperf equivalent to idle_table_cycle. This ticket will attempt to add that feature to workgen. The feature enables a parallel thread that creates and drops tables, and issues warning (or aborts the test) if any of the creates or drops take longer than a configurable duration.