Hazard site query ordering, part 2

A related follow-up to https://github.com/gem/oq-engine/pull/1241.

At the time this was submitted, there is no evidence of a problem. But, this change certainly couldn't hurt (although I haven't run tests on this yet) and could very well prevent some future problem.

Update: CI build queued: http://ci.openquake.org/job/zdevel_oq-engine/94/