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.
well, I think that to be sure we would have to check when the entry log is added. I am not sure about the performance impact of that though. 

But thinking about the logic, the scheduled task would check every time a new entry is added for the size of the execution log. If it's above 0.9 percent, than it would add the last log entry telling that the memory limit was hit and would set a flag telling that the execution log is closed. Then the file loader could check for that flag on each file claim, and abort the processing if the flag is true. What do you think?