Feature: [Linkgraph] Prioritize faster routes for passengers, mail and express cargo

Passengers usually prefer fast paths to short paths.
Average travel times of links are updated in real-time for use in Dijkstra's algorithm,
and newer travel times weigh more, just like capacities.

src/saveload/saveload.h

@@ -337,6 +337,7 @@ enum SaveLoadVersion : uint16 {
 
 	SLV_TABLE_CHUNKS,                       ///< 295  PR#9322 Introduction of CH_TABLE and CH_SPARSE_TABLE.
 	SLV_SCRIPT_INT64,                       ///< 296  PR#9415 SQInteger is 64bit but was saved as 32bit.
+	SLV_LINKGRAPH_TRAVEL_TIME,              ///< 297  PR#9457 Store travel time in the linkgraph.
 
 	SL_MAX_VERSION,                         ///< Highest possible saveload version
 };