Add support for adjusting the thread scheduling policy

[gittyupagain]

In order to properly get thread priorities working on Unix-like operating systems, a realtime scheduler needs to be used. This PR provides the ability to adjust the scheduler to be used when creating a new thread.

[jdonald]

All looks good here. That CAS loop can never really busy-wait even if two threads attempt to set priority at the same time, all tests still pass, Windows behavior is unaffected, and we now have a straightforward path to test with SCHED_FIFO.