Change SharesMap form std::map to btree map.

src/station_base.h

@@ -19,6 +19,7 @@
 #include "industry_type.h"
 #include "linkgraph/linkgraph_type.h"
 #include "newgrf_storage.h"
+#include "3rdparty/cpp-btree/btree_map.h"
 #include <map>
 #include <vector>
 
@@ -36,7 +37,7 @@ static const byte INITIAL_STATION_RATING = 175;
  */
 class FlowStat {
 public:
-	typedef std::map<uint32, StationID> SharesMap;
+	typedef btree::btree_map<uint32, StationID> SharesMap;
 
 	static const SharesMap empty_sharesmap;