SPARK-1775: Unneeded lock in ShuffleMapTask.deserializeInfo

This was used in the past to have a cache of deserialized ShuffleMapTasks, but that's been removed, so there's no need for a lock. It slows down Spark when task descriptions are large, e.g. due to large lineage graphs or local variables.

Author: Sandeep <sandeep@techaddict.me>

Closes #707 from techaddict/SPARK-1775 and squashes the following commits:

18d8ebf [Sandeep] SPARK-1775: Unneeded lock in ShuffleMapTask.deserializeInfo This was used in the past to have a cache of deserialized ShuffleMapTasks, but that's been removed, so there's no need for a lock. It slows down Spark when task descriptions are large, e.g. due to large lineage graphs or local variables.