Raise on 'order' being sent to find_each as .id is going to used regardless

This PR includes a suggest additional find_each param *:allow_unused_order*
In short, the idea is to raise an exception when this flag is not being sent, rather than logging this to the log file.
Specifically, I suggest this because I believe that in many cases, find_each / find_in_batches are actually used in background (resque-like) or Rails runner code and are therefore hard to come across when logged to a file.