(PR#10203) Greedy CM algorithm

<URL: http://rt.freeciv.org/Ticket/Display.html?id=10203 >

Bugfix.  It's now consistently faster, and always correct.  In over 50 games, the BB code takes 
75% of the time of the DP code, pretty much on every query.

Incidentally, it has revealed that the old dynamic programming code actually has a bug when 
you disband a unit in a city: the cache doesn't get cleared, so it ends up returning a result for 
the city still having the unit.  The branch-and-bound code doesn't cache anything, so it 
doesn't suffer from that.