Calling BKE_nodetree_attribute_hint_add from multiple threads still was not safe before..
One issue was that context_map embedded its values directly. So when context_map grows, all NodeUIStorage would move as well.
I could patch around that by using std::unique_ptr in a few places, but that just becomes too complex for now.
Instead I simplified the locking a bit by adding just locking a mutex in NodeTreeUIStorage all the time while an attribute hint is added.

This is not actually much worse performance wise than before, because we did lock a similar mutex before, just for a slightly shorter time.

Going forward it might be a good idea to refactor this a bit more. Maybe the evaluator could remember all the attribute hints and then move them to NodeTreeUIStorage in one go.