The numeric id is only available before the storage entry is deleted, so
get it at that time.

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