Early abort for relocate and Or-opt moves

[jcoupey]

Commit 696aa4b introduced an early abort when evaluating a relocate step for the TSP local search, saving quite a few loops. It's based on the fact that we can be sure that a bunch of potential gains won't be better than the one already known, even without complete evaluation.

It seems like we could port this principle to the CVRP local search phase, both for relocate and or-opt moves, respectively here and here.

The good part is that the necessary values are already pre-computed in solution_state::node_gains and solution_state::edge_gains.

[jcoupey]

Done in #155.