[SPARK-2189][SQL] Adds unregisterTempTable API

This PR adds an API for unregistering temporary tables. If a temporary table has been cached before, user can unpersist it as well.
But indeed I think this API is not intuitive. Another choice is to ask users to uncache the table explicitly before call `dropTempTable`, and simply don't care about caching in `dropTempTable`. But this calling sequence is not thread-safe, plus it's verbose and error prone from user's perspective.

The ultimate solution should be another refinement of caching semantics: namely making cached columnar RDDs "reference counted", a cached columnar RDD shouldn't be removed as long as there's at least one temporary table uses it.

Maybe for now, we can just unpersist the cache if any and accept the risk of losing cache of multiple tables?