Implement a shared thread pool

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Some modules start their own threads currently.
Others use an ExecutorService.

We need a standard way of running background tasks that does not 
compromise performance (tasks should not block because there are no 
threads available), but at the same time reuse threads whenever possible 
across tasks, reduce the number of threads allocated by default, etc.

Original issue reported on code.google.com by [email protected] on 14 Mar 2010 at 12:30