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/linkgraph_sl.cpp

@@ -33,6 +33,7 @@ public:
 	inline static const SaveLoad description[] = {
 		    SLE_VAR(Edge, capacity,                 SLE_UINT32),
 		    SLE_VAR(Edge, usage,                    SLE_UINT32),
+		SLE_CONDVAR(Edge, travel_time_sum,          SLE_UINT64, SLV_LINKGRAPH_TRAVEL_TIME, SL_MAX_VERSION),
 		    SLE_VAR(Edge, last_unrestricted_update, SLE_INT32),
 		SLE_CONDVAR(Edge, last_restricted_update,   SLE_INT32, SLV_187, SL_MAX_VERSION),
 		    SLE_VAR(Edge, next_edge,                SLE_UINT16),

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
 };