Revert to previous OOTB defaults for num_reader_threads and num_writer_threads

As discussed with various folks, please revert to the old mechanism for how num_reader_threads and num_writer_threads is calculated. I know this mechanism is complicated and not simple to explain, however, I think it's best to revert so we are backward compatible. If the desire is to tweak the way this is done, I think that's probably ok, but in my head the simplest approach it to revert to the way we do it in 6.0.