Linkgraph: Changes to job scheduling algorithm.

This is to improve responsiveness of link graph updates, whilst
avoiding being blocked waiting for updates to complete.
Previously, large numbers of cheap jobs resulted in poor responsiveness
as it took a long time for jobs to cycle round.

Add 'linkgraph' debug category.

src/linkgraph/linkgraph.h

@@ -15,6 +15,7 @@
 #include "../core/pool_type.hpp"
 #include "../core/smallmap_type.hpp"
 #include "../core/smallmatrix_type.hpp"
+#include "../core/bitmath_func.hpp"
 #include "../station_base.h"
 #include "../cargotype.h"
 #include "../date_func.h"
@@ -523,6 +524,11 @@ public:
 	NodeID AddNode(const Station *st);
 	void RemoveNode(NodeID id);
 
+	inline uint CalculateCostEstimate() const {
+		uint64_t size_squared = this->Size() * this->Size();
+		return size_squared * FindLastBit(size_squared * size_squared); // N^2 * 4log_2(N)
+	}
+
 protected:
 	friend class LinkGraph::ConstNode;
 	friend class LinkGraph::Node;