Add: estimate the new ticks_playing field for older servers (#294)

This field was added in Game Info v7, and although the master-server
doesn't (need to) support this, for simplicity it is easier if it
does calculate it with a rough estimate.

master_server/openttd/receive.py

@@ -122,6 +122,7 @@ def receive_PACKET_UDP_SERVER_RESPONSE(source, data):
                 "map_height",
                 "map_type",
                 "is_dedicated",
+                "ticks_playing",
             ]
         }
 
@@ -163,6 +164,8 @@ def receive_PACKET_UDP_SERVER_RESPONSE(source, data):
             payload["map_type"], data = read_uint8(data)
             payload["is_dedicated"], data = read_uint8(data)
 
+        payload["ticks_playing"] = max(0, (payload["game_date"] - payload["start_date"]) * 74)
+
         if len(data) != 0:
             raise PacketInvalidData("more bytes than expected")