Add a test for hql() in HiveQuerySuite.
Add a test for singleton set command in SQL.
WIP in changing SetCommand to take two Options (for different semantics of SETs).
Clean up semantics of several cases of SET.

- Add planner & physical plan for SET so that the pipeline propagates it
  cleanly.
- For HiveContext: make SQLConf and HiveConf equal after initialization
  of the context.