This uses the same fetching as normal invisibility timeout except with an added background update to update the FetchedAt time regularly
This is disabled by default and must be set in PostgreSqlStorageOptions SlidingInvisibilityTimeout to enable it's usage.
Fixed two issues with tests where tests were asserting against record.FetchedAt but the record returned is the database column names so should be record.fetchedat

… timeouts are enabled to reduce server load if not enabled

Added additional documentation to SlidingInvisibilityTimeout to indicate that this and IsLightweightServer are not compatible

…ityTimeout to enable/disable using sliding timeout and maintain existing InvisibilityTimeout setting for both sliding and existing invisibility timeout options.