Call the post-processor when the allocation is "fixed" (i.e., nothing
changes anymore), instead of having a final loop across all UEs that
might potentially be costly.

In order to calculate the PF metric, note that the statistics "reset" is
moved into pf_dl() instead of the post-processing loop, as it has to be
done on each slot.

See also: dd980302 ("Refactor to call post-processor in place")