Calling an API function after the node panels patch does not internally
tag the node tree with `NTREE_CHANGED_INTERFACE` any more, because the
node tree is not directly accessible from `bNodeTreeInterface`. Before
node panels the API functions for interfaces could tag the tree directly
for later update consideration, which now requires explicit tagging
calls.
The fix is to add a flag and mutex directly to `bNodeTreeInterface`, so
API methods can tag after updates. This mostly copies runtime data
concepts from `bNodeTree`. The `ensure_interface_cache` method is
equivalent to `ensure_topology_cache` and should be called before
accessing `interface_inputs` and similar cache data.
