event: add function BlockingCall()

Replaces io_thread_call().  This approach is more generic and easier
to use due to std::function.

src/IOThread.cxx

@@ -115,53 +115,3 @@ io_thread_inside(void)
 {
 	return io.thread != NULL && g_thread_self() == io.thread;
 }
-
-struct call_data {
-	GThreadFunc function;
-	gpointer data;
-	bool done;
-	gpointer result;
-};
-
-static gboolean
-io_thread_call_func(gpointer _data)
-{
-	struct call_data *data = (struct call_data *)_data;
-
-	gpointer result = data->function(data->data);
-
-	io.mutex.lock();
-	data->done = true;
-	data->result = result;
-	io.cond.broadcast();
-	io.mutex.unlock();
-
-	return false;
-}
-
-gpointer
-io_thread_call(GThreadFunc function, gpointer _data)
-{
-	assert(io.thread != NULL);
-
-	if (io_thread_inside())
-		/* we're already in the I/O thread - no
-		   synchronization needed */
-		return function(_data);
-
-	struct call_data data = {
-		function,
-		_data,
-		false,
-		nullptr,
-	};
-
-	io.loop->AddIdle(io_thread_call_func, &data);
-
-	io.mutex.lock();
-	while (!data.done)
-		io.cond.wait(io.mutex);
-	io.mutex.unlock();
-
-	return data.result;
-}