Return union of all Hive-partition RDDs for SharkEnv#unpersist() and MemoryMetadataManager#unpersist().

Also added Option[RDD[_]] in MemoryTable to represent an empty table.
Header comment for QueryBlock.
Disable cached, Hive-partitioned tables for now.
Update some comments in MemoryTable, fix small SharkSemanticAnalyzer compile bug…
Add an isHivePartitioned() fn to MemoryMetadataManager.

Helpful for testing.
Allow creation of cached tables using non-CTAS CREATE TABLE.