Yes, I see it and I meant to use it.
But after my reading of the related-code of it, I think it is not a reference-counter but a session-number-counter. :(

Hmmm, no code to increase it if a new reference to the session and decrease it if a reference is break.
Instead, SESSION_COUNTER is increased in new_session and decreased in close session.

It seems like a session-number-counter more than a reference-counter.

I could be wrong, I think it will be used only in unittest in current code. :)