It is basically a better way to achive the kind of pruning you get when
removing unplausible plans: just explore the first branch for a given
joint structure.
We can be sure the first is the best by sorting the branches by average
joint plan weight rather than plan weight.
It allows to get speedup even when searching for non-blocking plans,
which was very slow before as pruning could make some non-blocking plans
blocking. For 50-size cliques, removal of extra plans takes less than
one sec compared to about one minute for the previous revision!
