Add pause method to ThreadManager

This could be useful in cases where you want to pause operations when resource usage (e.g. memory) gets to a certain point, and maybe continue after some of the resource frees up.  
  
Need to add to ThreadManager:
- pause() method
- paused attribute
- a method that can be used to wait for an Event or Condition of becoming un-paused
- in stop method, set paused to False if needed