Options for engine node-ordering on launch

[thiell]

Today, the pending clients (Engine._clients) are stored in a set. This has been a wanted feature introduced in 1.5 or 1.6 to have better performance on randomized sets. However, it could be interesting to feed the launchpad in order. Indeed, this case is interesting when used on a very large cluster AND using a small fanout window.

We could implement some kind of engine policy rules for that and other policies (TBD).