WIP Deleting missing nodes during snap sync

[gezero]

This is a work in progress…

Before persisting a new value for a node during the heal step of snap sync, we check if there are some old nodes that should be deleted and delete those nodes from database.

Note that we need to touch each node of the tree that needs to be deleted.

We could batch the storage deletions from the flat database in one call for a whole account, but to delete the data from the bonsai tree we would still have to touch each node.

No tests here yet, sorry...