DataObjectModification.getModificationType() throws ISE on UNMODIFIED

This problem is handled by OVSDB's TransactCommandAggregator, which explicitly catches the ISE generated by DataObjectModification.getModificationType().

The ISE is generated because modified children of a SUBTREE_MODIFIED node include a MERGE operation which turned out to be an UNMODIFIED node – which Binding cannot map, as it has only DELETE/WRITE/SUBTREE_MODIFIED options.

Direct users usually handle this gracefully by ignoring the unmodified node when iterating over the nodes – but Binding would have to always filter modified nodes.

This poses an interesting lifecycle problem: the node should really be pruned from the DataTreeCandidate, so that if this is the only modification within a SUBTREE_MODIFIED parent, that parent would turn into UNMODIFIED, and cascade upwards.