Change: [Linkgraph] Allow job threads to be aborted early when clearing schedule (#8416)

When link graph jobs are cleared due to abandoning the game or exiting,
flag the job as aborted.
The link graph job running in a separate thread checks the aborted flag
periodically and terminates processing early if set.
This reduces the delay at game abandon or exit if a long-running job
would otherwise still be running.

src/linkgraph/linkgraphjob.cpp

@@ -38,7 +38,8 @@ LinkGraphJob::LinkGraphJob(const LinkGraph &orig) :
 		link_graph(orig),
 		settings(_settings_game.linkgraph),
 		join_date(_date + _settings_game.linkgraph.recalc_time),
-		job_completed(false)
+		job_completed(false),
+		job_aborted(false)
 {
 }
 
@@ -92,6 +93,11 @@ LinkGraphJob::~LinkGraphJob()
 	 * Accessing other pools may be invalid. */
 	if (CleaningPool()) return;
 
+	/* If the job has been aborted, the job state is invalid.
+	 * This should never be reached, as once the job has been marked as aborted
+	 * the only valid job operation is to clear the LinkGraphJob pool. */
+	assert(!this->IsJobAborted());
+
 	/* Link graph has been merged into another one. */
 	if (!LinkGraph::IsValidID(this->link_graph.index)) return;