network_engine: expire requests when clearing NE

This is to prevent any thread relying on network_engine's callback to block
forever just like the Dht::sendPing's callback.

src/network_engine.cpp

@@ -131,7 +131,7 @@ void
 NetworkEngine::clear()
 {
     for (auto& req : requests)
-        req.second->cancel();
+        req.second->setExpired();
     requests.clear();
 }