NIC switchover dates

I'm pretty sure that NSI started doing the NIC stuff in 1991, it
certainly wasn't 1992, but it might have been as early as 1990.
It was definitely before I left SURAnet.

It would be trivial to go back and see which IETF plenaries were
devoted to the initial CERFnet/AT&T/NSI presentations and then the
later thrashings of Scott Williams (who was the mgr at NSI in the
beginning of the project).