BUG/MINOR: tools: seed the statistical PRNG slightly better

BUG/MINOR: tools: seed the statistical PRNG slightly better

Thomas Baroux reported a very interesting issue. "balance random" would
systematically assign the same server first upon restart. That comes from
its use of statistical_prng() which is only seeded with the thread number,
and since at low loads threads are assigned to incoming connections in
round robin order, practically speaking, the same thread always gets the
same request and will produce the same random number.

We already have a much better RNG that's also way more expensive, but we
can use it at boot time to seed the PRNG instead of using the thread ID
only.

This needs to be backported to 2.4.