[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.
Hi, `SimpleDateFormat` is not thread-safe, so `def` should be used instead of `val`.