etcp: add node_changed callback chain + chatgui subscription

- etcp.h: add node_changed_cbks field to ETCP_CONN
- etcp_api.h/c: add etcp_conn_node_changed() + add/remove_node_changed_cbk
- etcp_connections.c: fire node_changed callbacks on pubkey mismatch
  (with DEBUG_WARN(GENERAL) logging source addr, peer_id, old/new pubkey)
- etcp.c: cleanup node_changed_cbks on connection close
- chat_conn_mgr.c: subscribe to node_changed — clear DB addresses,
  reset in-memory cache, close connections (deferred for current conn),
  post GUI_EVT_NODE_CHANGED
- gui_bridge.h: GUI_EVT_NODE_CHANGED=11
- gui_bridge_impl.cpp: handle GUI_EVT_NODE_CHANGED

tools/chatgui/transport/gui_bridge_impl.cpp

@@ -130,6 +130,14 @@ void GuiBridgeReceiver::processPost(int eventType, QByteArray data) {
     case GUI_EVT_STATUS_REFRESH:
         if (g_status_refresh_cb) g_status_refresh_cb((const char*)d, dlen);
         break;
+    case GUI_EVT_NODE_CHANGED:
+        if (dlen >= 8) {
+            uint64_t nodeId; memcpy(&nodeId, d, 8);
+            DEBUG_INFO(DEBUG_CATEGORY_DEBUG, "gui_bridge: NODE_CHANGED node=0x%016llx — pubkey changed, connection closed", (unsigned long long)nodeId);
+        } else {
+            DEBUG_WARN(DEBUG_CATEGORY_DEBUG, "gui_bridge: NODE_CHANGED data too short %d", dlen);
+        }
+        break;
     default:
         DEBUG_WARN(DEBUG_CATEGORY_DEBUG, "gui_bridge: unknown event type %d", eventType);
         break;