Fix #9928

- the file/folder's permission is now stored in the file cache
- BackGroundWatcher has been removed - this has meanwhile be replaced by occ files:scan which can be executed in a cron jobs
- increase version to trigger database migration

* Use "filesystem_cache_readonly" config setting, update comment in config.sample
* Use $this->cacheActive to cache config setting
* Add public Scanner::setCacheActive() to set $cacheActive programmatically

so that descendant classes can access them.

* test case added to avoid adding them later

Added extra code to handle the case of Oracle which saves empty strings
as null values.