Added a simple version numbering system for backup format

[amitlan]

With this change, pg_rman will write a "version" number to a designated file when initiliazing the backup directory.

From now on, every change to the backup file format is to be accompanied by an "increment" of backup_catalog_version.

NOTE: This system needs to be extended for considering changes such as adding a new members to structs like BackupPageHeader