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'm also seeing points of failure at the String::regexp_replace() lines following https://github.com/pkp/pkp-lib/blob/ojs-dev-2_4/classes/mail/Mail.inc.php#L483.

I've set a memory_limit of 20M for testing.  In my last test I broke out of processing at 15.1M (after 4 monthly logs), and still hit the 20M memory limit with a serialized execution log size of 1.3M.