Allow apps to control via a hook to skip adding/removing a file during filescan
Rename variable to something more appropriate.
Allow the default behavior to come from the config.
Use 'filesystem_check_enable' as a config option.
Prevent apps from affecting the scanner via hook.
Updates per comments on PR:

* 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
Missed one.
Merge pull request #8607 from owncloud/filescan_app_hook

Allow apps to control via a hook skipping add/remove a file during filescan