In 336c856e52d211aad6b40d29986264f3277a1327 : * `channel_get_var_cell_handler()` and `channel_get_cell_handler() `looks very similar * `channel_write_cell()`, `channel_write_packed_cell()` and `channel_write_var_cell()` is mostly duplicated code. * so are `channel_queue_cell()` and `channel_queue_var_cell()` We should refactor to reduce the code duplication.