Properly cleanup DAGScheduler on job cancellation.

Previously, when jobs were cancelled, not all of the state in the
DAGScheduler was cleaned up, leading to a slow memory leak in the
DAGScheduler.  As we expose easier ways to cancel jobs, it's more
important to fix these issues.

This commit also fixes a second and less serious problem, which is that
previously, when a stage failed, not all of the appropriate stages
were cancelled.  See the "failure of stage used by two jobs" test
for an example of this.  This just meant that extra work was done, and is
not a correctness problem.

This commit adds 3 tests.  “run shuffle with map stage failure” is
a new test to more thoroughly test this functionality, and passes on
both the old and new versions of the code.  “trivial job
cancellation” fails on the old code because all state wasn’t cleaned
up correctly when jobs were cancelled (we didn’t remove the job from
resultStageToJob).  “failure of stage used by two jobs” fails on the
old code because taskScheduler.cancelTasks wasn’t called for one of
the stages (see test comments).

This should be checked in before #246, which makes it easier to
cancel stages / jobs.