Cache country lookup result in PeerInfo class

The country lookup happens quite often when "Resolve peer countries"
option is enabled.

src/base/bittorrent/peerinfo.h

@@ -102,6 +102,10 @@ namespace BitTorrent
         qreal m_relevance = 0;
         QString m_flags;
         QString m_flagsDescription;
+
+#ifndef DISABLE_COUNTRIES_RESOLUTION
+        mutable QString m_country;
+#endif
     };
 }