lib/curl/Request: require the caller to explicitly register the request

This allows constructing an instance in any thread, and register it
inside the IOThread later.

src/input/plugins/CurlInputPlugin.cxx

@@ -373,6 +373,8 @@ CurlInputStream::InitEasy()
 	request_headers.Clear();
 	request_headers.Append("Icy-Metadata: 1");
 	request->SetOption(CURLOPT_HTTPHEADER, request_headers.Get());
+
+	request->Start();
 }
 
 void