Server cache validation policy. Specifies when server-side cache entries are validated.
With expiration-based validation, source materials and vignettes are periodically checked to see whether they have changed. With catalog-based validation, source images are checked only after the
catalog::TimeStamp value changed.
Catalog-based validation is recommended when both material and vignette catalogs are used. Expiration-based validation should be used when vignettes are referenced in Image Rendering requests directly by path.
Enum. 0 to select expiration-based validation. 1 to select catalog-based cache validation.
default::CacheValidationPolicy if not defined or if empty.