Pause the game instead of blocking when link graph jobs lag.

Check if the job is still running one date fract tick before it
is due to join and if so pause the game until its done.
This avoids the main thread being blocked on a thread join.
Show if pause is due to link graph job in status bar, update network
messages.
This does not apply for network clients.

src/linkgraph/linkgraphschedule.h

@@ -57,6 +57,7 @@ public:
 	static void Clear();
 
 	void SpawnNext();
+	bool IsJoinWithUnfinishedJobDue() const;
 	void JoinNext();
 	void SpawnAll();
 	void ShiftDates(int interval);