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.
Possible, though less likely because the new increase in the execute log would have to be substantially bigger than any previously seen increase.  Originally I was strictly using memory_get_usage() to track the usage growth.  I'm ambivalent about which is the better approach.