Many changes to ensure better 2nd recovery if 2nd failure happens while

recovering from 1st failure
- Made the scheduler to checkpoint after clearing old metadata which
  ensures that a new checkpoint is written as soon as at least one batch
  gets computed  while recovering from a failure. This ensures that if
  there is a 2nd failure while recovering from 1st failure, the system
  start 2nd recovery from a newer checkpoint.
- Modified Checkpoint writer to write checkpoint in a different thread.
- Added a check to make sure that compute for InputDStreams gets called
  only for strictly increasing times.
- Changed implementation of slice to call getOrCompute on parent DStream
  in time-increasing order.
- Added testcase to test slice.
- Fixed testGroupByKeyAndWindow testcase in JavaAPISuite to verify
  results with expected output in an order-independent manner.