Stop objectifying various trackers, caches, etc.

core/src/main/scala/spark/BitTorrentBroadcast.scala

core/src/main/scala/spark/Cache.scala

core/src/main/scala/spark/CacheTracker.scala

core/src/main/scala/spark/ChainedBroadcast.scala

core/src/main/scala/spark/CoGroupedRDD.scala

core/src/main/scala/spark/DAGScheduler.scala

core/src/main/scala/spark/DfsBroadcast.scala

core/src/main/scala/spark/DiskSpillingCache.scala

core/src/main/scala/spark/Executor.scala

core/src/main/scala/spark/JavaSerializer.scala

core/src/main/scala/spark/KryoSerialization.scala

core/src/main/scala/spark/LocalFileShuffle.scala

core/src/main/scala/spark/LocalScheduler.scala

core/src/main/scala/spark/MapOutputTracker.scala

core/src/main/scala/spark/RDD.scala

core/src/main/scala/spark/Serializer.scala

core/src/main/scala/spark/SerializingCache.scala

core/src/main/scala/spark/ShuffledRDD.scala

core/src/main/scala/spark/SparkContext.scala

core/src/main/scala/spark/SparkEnv.scala