This API should be seen as a replacement for (roughly) atomic<shared_ptr<T>>, a mutex+shared_ptr<T> (which is how atomic<shared_ptr> is implemented) or places where we frequently take a mutex just to observe an object AND we release the mutex prior to acting on the observation.

This should support:

• Non-blocking reads: readers never block behind writers or other readers.
• Reading stale values: readers may hold a reference to an earlier version of the object.
• Writers to create a new version without blocking on readers to retire.
• For efficiency, we should avoid having readers write to shared cache lines.