verify that IOThreadPools get shut down (#635)
verify that IOThreadPools get shut down (#635)

Motivation:

Not shutting down IOThreadPools leads to thread leaks so we should
verify they're actually shut down.

Modifications:

add a check that we're shutting down

Result:

fewer thread leaks
apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio