When a child watcher is fired, it can happen that when the children are
read, a deleted node is already added back. So, from the list of children,
it's as if nothing has changed. However, it could be important for an app
to know in this case that something has changed. I could include some
metadata per node and read every child node each. However, reading the data
for every child could be expensive. What's best way to handle this case?