NumStackOrder=3.
When it was originally chosen, the 3 cached sizes were 8k, 16k, 32k.
Now that the min stack is 2k, the 3 cached sizes are 2k, 4k, 8k.
16k and 32k are no longer cached at all.
If it made sense to cache them before, it probably still makes sense to cache them, in
which case NumStackOrder should change to 5.

Not for Go 1.4 unless we can demonstrate a real problem. So far no one seems to be
complaining.