Send multicasts in random order.

This should not affect most users, but on large networks it should cause service
announcements to work a lot better. This is the result of a prolonged discussion
with a user about the visibility of game servers on a large network. The old
multicast algorithm was de-facto randomized due to its distributed nature, while
the new algorithm is more deterministic. This will restore some randomization
beyond limit-overflow conditions.

It won't affect small networks at all.