CLOUD: Add RequestIdPair struct

Can be used with Callback<T> (means it's still type safe). It's used to
pass not only Request id to user's callback, but also a value user
wanted.

void *data field is removed from RequestInfo.

backends/networking/curl/curljsonrequest.h

@@ -37,7 +37,7 @@ class CurlJsonRequest: public CurlRequest {
 	char *getPreparedContents();
 
 public:
-	CurlJsonRequest(Common::BaseCallback<> *cb, const char *url);
+	CurlJsonRequest(DataCallback cb, const char *url); //TODO: use some Callback<JSON> already
 	virtual ~CurlJsonRequest();
 
 	virtual bool handle();