[SPARK-1021] Defer the data-driven computation of partition bounds in so...

...rtByKey() until evaluation.

Author: Erik Erlandson <eerlands@redhat.com>

Closes #1689 from erikerlandson/spark-1021-pr and squashes the following commits:

50b6da6 [Erik Erlandson] use standard getIteratorSize in countAsync
4e334a9 [Erik Erlandson] exception mystery fixed by fixing bug in ComplexFutureAction
b88b5d4 [Erik Erlandson] tweak async actions to use ComplexFutureAction[T] so they handle RangePartitioner sampling job properly
b2b20e8 [Erik Erlandson] Fix bug in exception passing with ComplexFutureAction[T]
ca8913e [Erik Erlandson] RangePartition sampling job -> FutureAction
7143f97 [Erik Erlandson] [SPARK-1021] modify range bounds variable to be thread safe
ac67195 [Erik Erlandson] [SPARK-1021] Defer the data-driven computation of partition bounds in sortByKey() until evaluation.