MINOR: sample: make the rand() sample fetch function use the statistical_prng

MINOR: sample: make the rand() sample fetch function use the statistical_prng

Emeric noticed that producing many randoms to fill a stick table was
saturating on the rand_lock. Since 2.4 we have the statistical PRNG
for low-quality randoms like this one, there is no point in using the
one that was originally implemented for the purpose of creating safe
UUIDs, since the doc itself clearly states that these randoms are not
secure and they have not been in the past either. With this change,
locking contention is completely gone.