p2p: disable gray list housekeeping when an exclusive node is given

Exclusive nodes may be used for privacy reasons, and thus we don't
want to connect to other nodes, even for checking connectivity.

See https://github.com/monero-project/monero/issues/2346

src/p2p/net_node.inl

@@ -1951,6 +1951,8 @@ namespace nodetool
   template<class t_payload_net_handler>
   bool node_server<t_payload_net_handler>::gray_peerlist_housekeeping()
   {
+    if (!m_exclusive_peers.empty()) return true;
+
     peerlist_entry pe = AUTO_VAL_INIT(pe);
 
     if (!m_peerlist.get_random_gray_peer(pe)) {