Major Description
We removed this for two reasons:
- nothing was using it
- the map is "unsafe" if a codec tried to write to it during in-place dv update.
But Adrien has a real use case (LUCENE-5914), and I think we can just add some safety for the updates case (e.g. if the map is unmodifiable then the trap will not exist, any put() will throw exception). In general, we should have more safety in SI anyway (diagnostics map, too).