PartialMmr remove and untrack should return changes to the nodes

[hackaugusto]

The PartialMmr is used by the client, which needs to persist the data to reconstruct it on restarts. Currently, only the addition of elements is being tracked, which allows the client to reconstruct the PartialMmr, but there is not mechanism returning the elements which have been removed, to clean up unnecessary state from the client. additional context