TEZ-1637. Improved shuffle error handling across NM restarts.
Contributed by Rajesh Balamohan.
TEZ-1479. Disambiguate (refactor) between
ShuffleInputEventHandlers and Fetchers. (sseth)
TEZ-1665. DAGScheduler should provide a priority range instead of an exact priority (bikas)jps
TEZ-1479 addendum. Revert accidental change from previous commit.
(sseth)