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.

test/run_input.cxx

@@ -175,9 +175,9 @@ class DumpRemoteTagHandler final : public RemoteTagHandler {
 
 public:
 	Tag Wait() {
-		const std::lock_guard<Mutex> lock(mutex);
+		std::unique_lock<Mutex> lock(mutex);
 		while (!done)
-			cond.wait(mutex);
+			cond.wait(lock);
 
 		if (error)
 			std::rethrow_exception(error);