replace trackers from resume data instead of adding them.

docs/manual.rst

@@ -2555,6 +2555,10 @@ replace it. If you want an immediate effect, you have to call
 If it is, it doesn't do anything. If it's not in the current set of trackers,
 it will insert it in the tier specified in the announce_entry.
 
+The updated set of trackers will be saved in the resume data, and when a torrent
+is started with resume data, the trackers from the resume data will replace the
+original ones.
+
 
 add_url_seed() remove_url_seed() url_seeds()
 --------------------------------------------

src/torrent.cpp

@@ -3512,6 +3512,7 @@ namespace libtorrent
 		lazy_entry const* trackers = rd.dict_find_list("trackers");
 		if (trackers)
 		{
+			m_trackers.clear();
 			int tier = 0;
 			for (int i = 0; i < trackers->list_size(); ++i)
 			{