refactor: improve criteria for choosing peers to save to resume (#6922)

* fix: only consider peers with listening ports interesting

It's a waste of space to store peers we cannot connect to (because we don't know what their listening port is).

* fix: don't consider peer as interesting just because we are currently connected to it

For example, it's possible for a peer to be "in-use" and "banned" at the same time, albeit just for a very short while.

* code review: assert that peer port is not empty

libtransmission/peer-mgr.cc

@@ -1465,14 +1465,15 @@ namespace get_peers_helpers
 
 [[nodiscard]] bool is_peer_interesting(tr_torrent const* tor, tr_peer_info const& info)
 {
-    if (tor->is_done() && info.is_upload_only())
+    TR_ASSERT(!std::empty(info.listen_port()));
+    if (std::empty(info.listen_port()))
     {
         return false;
     }
 
-    if (info.is_in_use())
+    if (tor->is_done() && info.is_upload_only())
     {
-        return true;
+        return false;
     }
 
     if (info.is_blocklisted(tor->session->blocklist()))