Add warning/max object limits

CSEs have wanted more visibility into sibling and object size explosions for a while. A warning threshold would make sure that the logs can be used to pinpoint or event prevent size and sibling number explosions.

We have also discussed putting hard limits on size and sibling number. Writes would fail for very large objects or objects with too many siblings. This should only be used as a last resort to prevent a cluster from experiencing severe performance problems that can result in cascaded deaths even