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.
Another brutal operation is https://github.com/pkp/pkp-lib/blob/ojs-dev-2_4/classes/mail/SMTPMailer.inc.php#L141, where we take the string and explode it back into an array.  Now the execution log has at least three copies in memory: the initial array, the string version, and the exploded string array.