Performance improvements for level crossing updating.
DistanceTo/Travellers now support a maximum search distance to limit work done.
Level crossing activations now only scan up to 2000m down the track.
git-svn-id: http://svn.uktrainsim.com/svn/openrails/trunk@673 a9f3cfff-6c41-7446-b0ca-052bd761e6eb