Simulate large scale query planning in TpcdsCpuCostPlannerTest

Querying against large scale database is a good way to test Impala. However, it is impractical to do in single node development machine.

Frontend testing does not actually run the test query in backend executor and can benefit from simulated large scale test cases. This can be done by either instrumenting the CatalogD metadata loading code or the COMPUTE STATS query to multiply numRows, numNull, numTrues, and numFalses to a scale constant. We can start by hacking TpcdsCpuCostPlannerTest to simulate 1TB TPC-DS query planning.