Updates current_src cache corresponding to save_dir. All the copies of old info are going to be incorrect. If save_async in the save_dir is not finished, calling this results undefined behavior.
Dochy files can be safely removed.
saves a Dochy file.
non-blocking version. spawns 1 OS thread. Saves clone of the RootObject, and cloning is very fast because RootObject consists of Arcs. Actual copying occurs only when the RootObject is modified before the saving is finished, and only the parts to be modified are copied using Arc::make_mut.