sd-event: don't provide priority stability

Currently, we guarantee that if two event-sources with the same priority
fire at the same time, they're always dispatched in the same order. While
this might sound nice in theory, there's is little benefit in providing
stability on that level. We have no control over the order the events are
reported, hence, we cannot guarantee that we get notified about both at
the same time.

By dropping the stability guarantee, we loose roughly 10% Heap swaps in
the prioq on a desktop cold-boot. Krzysztof Kotlenga even reported up to
20% on his tests. This sounds worth optimizing, so drop the stability
guarantee.