Renamed stageIdToActiveJob to jobIdToActiveJob.

This data structure was misused and, as a result, later renamed to an incorrect name.

This data structure seems to have gotten into this tangled state as a result of @henrydavidge using the stageID instead of the job Id to index into it and later @andrewor14 renaming the data structure to reflect this misunderstanding.

This patch renames it and removes an incorrect indexing into it.  The incorrect indexing into it meant that the code added by @henrydavidge to warn when a task size is too large (added here https://github.com/apache/spark/commit/57579934f0454f258615c10e69ac2adafc5b9835) was not always executed; this commit fixes that.