Debug info on local search operators

[jcoupey]

For development/debugging, it would be useful to compute some stats on local search operators. Typically how many moves of each type are tested and how many are applied (maybe with associated gain). I don't think we have a sound way to decide how many of the tried moves are actually valid because most of them are usually discarded without even checking validity, based on the potential gain (or a lower bound of the gain).

[jcoupey]

At first I though about simply using static members for all operator classes for counts, but that won't work because of concurrency (else we'd have to wrap the updates around locks which is a no-go).

Probably the best way is to have each LocalSearch instance keep track of what is going on during its own run. The upside is that we can then have per-search stats.