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/debug.cpp

@@ -46,6 +46,7 @@ int _debug_gamelog_level;
 int _debug_desync_level;
 int _debug_yapfdesync_level;
 int _debug_console_level;
+int _debug_linkgraph_level;
 #ifdef RANDOM_DEBUG
 int _debug_random_level;
 #endif
@@ -75,6 +76,7 @@ struct DebugLevel {
 	DEBUG_LEVEL(desync),
 	DEBUG_LEVEL(yapfdesync),
 	DEBUG_LEVEL(console),
+	DEBUG_LEVEL(linkgraph),
 #ifdef RANDOM_DEBUG
 	DEBUG_LEVEL(random),
 #endif