fix performance issue reported by Junna

James,
I think this fixes the performance issue Junna saw. It reduces the number of attempts we make to grow a city that won't grow, and keeps going after failing to build a road that would be excessive but cannot be saved by straightening a diagonal.

I believe the second issue Junna reported was present prior to my latest round of patches, so it's possibly less urgent; I'm not totally sure that it's a simple matter of dividing rather than multiplying by the ticks per month factor, yet.

Philip