Reuse days rebase functions of DateTimeUtils in DaysWritable

The functions rebaseJulianToGregorianDays() and rebaseGregorianToJulianDays() were added by the PR https://github.com/apache/spark/pull/27915. The ticket aims to replace similar code in org.apache.spark.sql.hive.DaysWritable by the functions to:

1. Deduplicate code
2. The functions were better tested, and cross checked by reading parquet files saved by Spark 2.4