ffi: Support retrieving node ID from a clp::ffi::SchemaTree::NodeLocator.

[junhaoliao]

Request

If we can retrieve node ID from a clp::ffi::SchemaTree::NodeLocator, in subscriptions to schema tree node insertions (i.e. IrUnitHandler::handle_schema_tree_node_insertion) we will be able to compare the node's key name and store ID of the tree nodes in interest to improve log viewing experience.
"tree nodes in interest" - examples include log level & timestamp kv-pairs.

Possible implementation

Add a public method into the class like get_node_id().

[junhaoliao]

For now, clp-ffi-js can mitigate by maintaining a counter in the IrUnitHandler class.