Forbid speculative insertion into catalogs

The possibility of creating a deadlock due to exclusive-locking an insertion
point into an index, and subsequent innocently accessing the catalogs is spooky
and hard to reason about --- even if I get it right now, things may change in
the future. So simply forbid speculative index insertion of catalogs.

In passing, add extensive commentary about why we don't do better with BEFORE
ROW triggers.