Switch back to using asyncs for worker threads.  Fixes issue #61.

We still have test failures in parallel, but the exceptions
seem like they are well contained so as to not screw up the test harness.
Sometimes all pass, sometimes I see problems in v9e, v9f1, v9g, v8d, i3c.
Spurious deadlocks and timeouts, all.
Reactivate StressTests.  Tweak it so that it includes a raw test as well.