Avoid taking write lock on span creation

Here's a rough sketch of the idea:

- Use an atomic `Slab` (@hawkw has something brewing) for `span id => group`.
- Use an atomic skiplist of `(group, group state)` for `group => group state`. On a new span, if the group does not exist in the list, try to insert it. If the insert CAS fails, search `idle-recorders` again for the group. If it does not exist, try to add it again. Repeat until successful.

With this, I _believe_ the entire outer lock can go away, as all access should be through `&self`. We may still want to keep it for `force_synchronize` though.