Swapped in the use of CountDownLatch to replace the  ]various] barrier code in Master. Results in fewer Command classes, elimination of a couple of methods in ServiceImpl, and, most importantly a cleaner Master (especially the run method).
Eliminated the ComputeThread class in preference to the use of ExecutorService and Runnables. 
Boldly removed "synchronize" from yet another Worker method.
Miscellaneous edits, mostly to restore no-arg constructors to Command classes.