Make it possible to customise the unreduce operation
When chaining an update, we want to use an uningest (unreduce) operation to remove the old value before adding the new value. Implement the necessary code to make this possible
Switch to using the unreduce technique for chained queries
Comment out some code that wasn't working (we'll need a fix for this eventually)
Fix some confusion over the action of re-reduce
Fix the behaviour of unreduce count to match that of reduce count