thread/Cond: add wait() overload which takes a unique_lock<>

Just like std::condition_variable, which however has no way to specify
the std::mutex directly.

src/storage/plugins/CurlStorage.cxx

@@ -124,9 +124,9 @@ public:
 	}
 
 	void Wait() {
-		const std::lock_guard<Mutex> lock(mutex);
+		std::unique_lock<Mutex> lock(mutex);
 		while (!done)
-			cond.wait(mutex);
+			cond.wait(lock);
 
 		if (postponed_error)
 			std::rethrow_exception(postponed_error);