[SPARK-10177] [SQL] fix reading Timestamp in parquet from Hive

We misunderstood the Julian days and nanoseconds of the day in parquet (as TimestampType) from Hive/Impala, they are overlapped, so can't be added together directly.

In order to avoid the confusing rounding when do the converting, we use `2440588` as the Julian Day of epoch of unix timestamp (which should be 2440587.5).

Author: Davies Liu <davies@databricks.com>
Author: Cheng Lian <lian@databricks.com>

Closes #8400 from davies/timestamp_parquet.