Change LinkGraph::EdgeMatrix to a sparse storage format

Use a btree_map instead of a SmallMatrix.
This avoids excessive storage requirements for large graphs,
and overly expensive edge matrix resize operations.
Remove next_edge iteration mechanism.

src/station.cpp

@@ -114,7 +114,7 @@ Station::~Station()
 		for (NodeID node = 0; node < lg->Size(); ++node) {
 			Station *st = Station::Get((*lg)[node].Station());
 			st->goods[c].flows.erase(this->index);
-			if ((*lg)[node][this->goods[c].node].LastUpdate() != INVALID_DATE) {
+			if (lg->GetConstEdge(node, this->goods[c].node).LastUpdate() != INVALID_DATE) {
 				st->goods[c].flows.DeleteFlows(this->index);
 				RerouteCargo(st, c, this->index, st->index);
 			}