graph-store: emit affected descended nodes in %remove-nodes
graph-store: emit affected descended nodes in %remove-nodes

When %remove-nodes is sent as an update, it may refer to a parent node,
in which case the deletion of all the parent's children is implied. If
we hold onto references to nodes outside of graph-store, we are unable
to tell which of our references were affected, other than iterating over
every reference to check if one of the deleted indices is a prefix of
the reference. This is obviously undesirable, so we emit all of the
nodes we deleted, including the nodes deleted implicitly.

ngzax/urbit,urbit/urbit,urbit/urbit,urbit/urbit,ngzax/urbit,urbit/urbit,urbit/urbit,ngzax/urbit,urbit/urbit,ngzax/urbit,urbit/urbit,ngzax/urbit,ngzax/urbit,ngzax/urbit