Fix issue #428

Added new performance.txt files for each test. Added delta operation count to performance logs. Fixed possible negative values on processing GetOperationCountCommand.

This should fix issue #428.
In itself this method is a bit confusing because when it is created this class is responsible of writing to file. Which is perfectly fine with me.

But.. after it is created it isn't responsible anymore to write to its own file and the enclosing structure takes care of it.

So give him full responsibility or give him no responsibility. But don't give him partial responsibility.

So move the writeMethods to the TestStats and let it be self contained.