feat: Concurrent safety for ordered maps

[otaxhu]

Using v2.

I saw in the source code that there is a lot of critical sections where race conditions can happen, this can be a feature that doesn't change the current API, more like a internal feature.

Have a great day.

[elliotchance]

That's by design, it doesn't promise to be safe for concurrent operations. Applications will have to provide locking in the way that makes sense for their respective use cases.