pom: Use Hadoop 2.4.1
ArrayLists instead of LinkedLists to store elements in Operators
Sort operator: Regular Collections.sort instead of PriorityList

Hoping that this frees buffers on previous tasks (as the input is
consumed faster than when each element is inserted into a regular
list that's sorted later instead of insertion sort)..
Removed legacy code (for transferring groups over channels)
Scheduling changes: round-robin usage of machines + fixes