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 think that the flag to check if the execution log is full can be protected, so each subclass would have access to it already. The way I propose, would solve the memory limit to all task subclasses. The way it is now solves only for tasks that inherit from the FileLoader... What each task do when the execution log is full can be each class responsibility.