[MAYBE: 1 of 2] use data-cid vs cid as it needs to be an attr (see cache-cln-2 for the other)

This is one of two PRs for the same issue. cid's of children of a parent that is being `remove()` d need to be unset from the .cache.events as well. 

This PR takes the 'larger' approach of using 'data-cid' set as an attribute so that it can be queried from the parent as context (then removed)

pros: data-cid is formalized and may be useful in other ways
cons: more code. overhead of getting and setting that attribute