Monitor memory usage with each file processed by FileLoader::executeActions()

Abort early if usage might exceed the PHP memory limit.  Resolves pkp/pkp-lib#245.
Yes, see https://github.com/pkp/pkp-lib/blob/ojs-dev-2_4/classes/scheduledTask/ScheduledTask.inc.php#L147
When we implode the array into a string, suddenly we need memory for both.  I have external logs which cause numerous notices (particularly "can't remove base_url" from non-OJS lines).