Slow callbacks no longer cause test failures, now they just emit a
diagnostic. Jenkins on MacOS frequently failed those tests.
We should never test for something that we can't fix.
The main reason for this merge proposal is the change to "public" API functions.
Use atomic counter to resolve data race on threadsRunning in callback.
Split up callbackShutdown() and scanShutdown() into two phases *Stop() and
*Cleanup(). The *Stop() functions signal worker threads, and wait for them to
exit. The *Cleanup() functions actually reclaim global resources.
These two mechanisms have couplings which are quite complex. I/O Intr scans
involve both scan lists and callbacks.
