Support for in-memory, Hive-partitioned tables.

Also added along the way:
* Creating tables without CTAS - e.g., CREATE TABLE test_cached (...) [PARTITIONED BY (...)]
* General cache policy interface and LRU implementation, since there isn't a good way to specify, from the CLI, which partitions to keep in memory.
* Table class, with MemoryTable and PartitionedMemoryTable subclasses, for containing RDD metadata.

Partition pruning and stats collection is still unsupported/untested. Also, some additions made a few files, especially MemoryStoreSinkOperator, more confusing. Planning to submit fixes for these two issues in another CL, since it might involve more refactoring.