[SPARK-2190][SQL] Specialized ColumnType for Timestamp

JIRA issue: [SPARK-2190](https://issues.apache.org/jira/browse/SPARK-2190)

Added specialized in-memory column type for `Timestamp`. Whitelisted all timestamp related Hive tests except `timestamp_udf`, which is timezone sensitive.
A not so clever solution is to cache golden answers of all timezones for these tests, then select the right version for the current build according to system timezone. (And hope developers don't build Spark SQL when traveling across timezones...)

@yhuai any thoughts?