Updated Rollups to :
  1) prefetch results via DKV (instead of DTask ) to cache results locally(common use-case is to start rollups for multiple columns in parallel and later to fetch them via DKV, often sequentially).
  2) added support for multiple parallel rollups request - second request will wait for the first in-progress to finish, (previously unimpl)
  3) unified histogram with rollups to be retrieved/launched by the same mechanism, histogram being optional part of rolups

Also added test.
Added checksum computation to Rollups. Did not connect to public interface yet.
Changed DTask.dinvoke to call H2O.submit instead of directly compute2 to keep fj betavior (in particular, onExceptionalCompletion to be called)
Updated RollupStats task to do cleanup in case of exception.
Updated Rollups test.