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.
I see the benefit of the generalization, but I wonder if it is worth the effort without an exception framework.

Incidentally, I think I've found the bigger problem.  I don't think the memory issue is so much https://github.com/pkp/pkp-lib/blob/ojs-dev-2_4/classes/scheduledTask/ScheduledTask.inc.php#L147 as it is https://github.com/pkp/pkp-lib/blob/ojs-dev-2_4/classes/scheduledTask/ScheduledTask.inc.php#L146.  I think this line makes a copy of the full executionLog array, which is substantially larger than the serialized string version.  There's a LOT of array overhead: http://nikic.github.io/2011/12/12/How-big-are-PHP-arrays-really-Hint-BIG.html

I'm testing `$logString = implode(PHP_EOL . PHP_EOL, $this->_executionLog);` there.