The o+i*p approach to visiting select cases in random
order stops being fair when there is some case that
is never ready.  If that happens, then the case that follows
it in the order gets more chances than the others.

In general the only way to ensure fairness is to make
all permutations equally likely.  I've done that by computing
one explicitly.

Makes the permutations correct for n >= 4 where
previously they were broken.  For n > 12, there's not
enough randomness to do a perfect job but this should
still be much better than before.

Fixes #1425.

R=r, ken2, ejsherry
CC=golang-dev
https://golang.org/cl/4037043