bot waypoints enhancement

When saving waypoints in the waypoint editor, a timestamp is now added to waypoints and links files (it looks like this: //WAYPOINT_TIME 2018-08-03 15:58:55). Here it is what happens when you manually instal external waypoints and links for the map Go but there is already an autogenerated links file (.cache) in your data/data/maps folder that overrides the newly installed links:

go.waypoints: timestamp
go.waypoints.cache: no timestamp
automatic links update, timestamp copied into go.waypoints.cache

go.waypoints: timestamp
go.waypoints.cache: timestamp
* if they match: waypoints and links are ok
* if they don't match: automatic links update, timestamp copied into go.waypoints.cache

Installing old waypoints:

go.waypoints: no timestamp
go.waypoints.cache: timestamp
automatic links update, timestamp removed from go.waypoints.cache

go.waypoints: no timestamp
go.waypoints.cache: no timestamp
keep current links just like before (they can't be good if waypoints are different from the old ones)