Lowers MAX_N_IN_THRESH to 24

It is extremely unlikely to be hit in practice, and has a large memory
impact because of the dynamic programming tables of compute_thresh_ops()
and compute_thresh_stacksize()

The dynamic programming tables of compute_thresh_ops() and
compute_thresh_stacksize() are sized for MAX_N_IN_THRESH, causing them
to contribute a substantial stack usage, especially with nested
thresh fragments.