[SPARK-1141] [WIP] Parallelize Task Serialization 

https://spark-project.atlassian.net/browse/SPARK-1141
@kayousterhout
copied from JIRA(design doc in JIRA is old, I'll update it later)
TaskSetManager.resourceOffer will return a TaskDescWithoutSerializeTask object , this object will be a half-copy of TaskDescrption exception _serializedTask ByteBffer, instead, it will contain a Task object and seriailze part inside TaskSetManager.resourceOffer will be moved to TaskSchedulerImpl's "Runnable" working thread which will be placed inside threadpool.