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/linkgraphjob.h

@@ -272,7 +272,7 @@ public:
 	LinkGraphJob() : settings(_settings_game.linkgraph), thread(NULL),
 			join_date_ticks(INVALID_DATE), start_date_ticks(INVALID_DATE), job_completed(false) {}
 
-	LinkGraphJob(const LinkGraph &orig);
+	LinkGraphJob(const LinkGraph &orig, uint duration_multiplier);
 	~LinkGraphJob();
 
 	void Init();