Improve naming and README files for MetaStore upgrade scripts

The Hive metastore upgrade scripts currently use the naming convention upgrade-X.Y.Z.<dbname>.sql, with the intention of indicating that this file handles the upgrade to version X.Y.Z from the previous release. This has caused confusion with users who wonder if upgrade script actually handles the upgrade to X.Y.Z from all earlier versions of Hive.

I propose changing the naming convention to upgrade-A.B.C-to-X.Y.Z.<dbname>.sql in order to clarify the intended usage of these scripts.