Thread-safe subclass mode

Right now, if C++ passes our subclass `unique_ptr` to a different thread, all bets are off. We should provide a configuration option to make a thread safe version using `Mutex` and `Arc` instead of `RefCell` and `Rc`. Maybe we can use [archery](https://docs.rs/archery/0.4.0/archery/).

Start by tracing how the subclass unique_ptr support currently uses RefCell and Rc. Review the proposed Mutex and Arc configuration and the linked archery crate, then identify the affected implementation and tests. Done means a selectable thread-safe subclass mode works when C++ passes the unique_ptr across threads.

Written by the indexing model from the issue text.