Using backported ConcurrentHashMap from JDK 8. 

This results in a speedup of about 10%.

For 10mil binary tree on 4 cores:
89287 : Total run time
   2 : Set WorkerJob & make Worker files
   22770 : Read Worker input files
   21522 : Worker Garbage Collection
   13545 : Computation
   541 : average per super step 
   31424 : Write Worker output files
Removed some unnecessary allocations
Fixed calling wrong constructor in make() method.