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.

Author: Kay Ousterhout <kayousterhout@gmail.com>

Closes #301 from kayousterhout/fixCancellation and squashes the following commits:

bd3d3a4 [Kay Ousterhout] Renamed stageIdToActiveJob to jobIdToActiveJob.
[SPARK-1385] Use existing code for JSON de/serialization of BlockId

`BlockId.scala` offers a way to reconstruct a BlockId from a string through regex matching. `util/JsonProtocol.scala` duplicates this functionality by explicitly matching on the BlockId type.
With this PR, the de/serialization of BlockIds will go through the first (older) code path.

(Most of the line changes in this PR involve changing `==` to `===` in `JsonProtocolSuite.scala`)

Author: Andrew Or <andrewor14@gmail.com>

Closes #289 from andrewor14/blockid-json and squashes the following commits:

409d226 [Andrew Or] Simplify JSON de/serialization for BlockId