Fix #760: update ShutdownDetector to latest Katana code

It's the Katana code with very minor changes:
- Changed namespace
- Debug.Writeline instead of trace (same as we did before)
- Put UnsafeIISMethods is same file (same as we did before)

All public Kudu tests pass, though I'm not sure how much they exercise this. Seems TestLogStreamAppShutdown is only run in Private Kudu.