Does not play nicely with `Threads.@spawn` or `Threads.@threads`

[chriselrod]

Two problems:

1. Possible race conditions. @tkf suggested a good fix here, which is to have a "job started" stated. Then, if the job hasn't started, ThreadingUtilitilies.wait can steal the task back (note that the overhead on this will be much higher than that of a proper work stealing implementation, which would use jumps).
2. ThreadingUtilities' tasks currently still spend some time before going to sleep. This gets in the way of Threads.@spawn and Threads.@threads, causing bad performance. It'd be great if there were something they could check to go to sleep immediately if someone used Thread.@spawn or Threads.@threads.