close tachyon buffer.
make both SparkCached and TachyonCached RDD go through the same partition prune code path.
address comments: better naming.
change TachyonTableRDD from ColumnarStruct to TablePartition to align code path with Spark cached RDD.
Merge pull request #123 from amplab/tachyon

1. Close buffer from Tachyon properly. 2. Make both spark cached and tachyon cached rdd use the same partition prune code path.