Invalidate config.php in OPcache on preferences save
Force invalidation of config.php regardless of last modified value

Some filesystems may be set not to record last modified dates if I remember correctly. Even if not, forcing shouldn’t be a problem and seems safest.
Invalidate data source and event files in OPcache on save
Merge pull request #2025 from DavidOliver/opcache

Handle PHP5.5's OpCache with flat files. Closes #2017