Remove the artifical cap on the number of workers

Remove the artifical cap on the number of workers
See #805.  This was here to catch bugs that resulted in an infinite
number of worker threads being created.  However, we can't put a
reasonable bound on the number of worker threads, because legitimate
programs may need to create large numbers of (probably blocked) worker
threads.  Furthermore, the OS probably has a bound on the number of
threads that a process can create in any case.