Add the ability to add two OpDebug objects together

To support the use of the OpDebug structure for tracking aggregate statistics inside transaction, we need a way to add together one OpDebug object to another. We should be able to do this by adding a OpDebug::add method that accepts an OpDebug object, and adds together any applicable fields for each object. Some fields that are not numeric in nature should not be added together, and should likely just retain the value of the original OpDebug object (the non-argument object).