event/Call, ...: use wait() with predicate

src/storage/plugins/CurlStorage.cxx

@@ -125,8 +125,7 @@ public:
 
 	void Wait() {
 		std::unique_lock<Mutex> lock(mutex);
-		while (!done)
-			cond.wait(lock);
+		cond.wait(lock, [this]{ return done; });
 
 		if (postponed_error)
 			std::rethrow_exception(postponed_error);