Remove overwrite for split-level of dynamic aux store

This will prevent attributes (really only ElementLinks) in the dynamic aux store from being split, making it more consistent with the layout of the rest of data in the file. It reduces the number of branches and minor (~0.3.%) storage savings due to better compression and less overhead (storage savings on affected containers are sizable). Since it is a 'technical' violation of frozen Tier0 policy (none of the transient data changes, but their persistent layout), I mark it as Draft until discussed in a Reconstruction meeting.