Runtime test to reproduce threadpool+ProcessExit issue

Do make test-process-exit to run
Fixes threadpool+ProcessExit problem

The threadpool was being shutdown too early, before the ProcessExit event was fired.
This led to bugs such as NLog not shutting down
(http://nlog-project.org/2011/10/30/using-nlog-with-mono.html).
Merge pull request #505 from roji/shutdown_flow

Fixed threadpool+ProcessExit problem in shutdown flow