Do not attempt to lock pg_shdescription if not superuser
Do not attempt to lock pg_shdescription if not superuser

RDS does not provide full Postgres superuser access and so
`SELECT .. FROM pg_shdescription FOR UPDATE` will fail because only
superusers have `UPDATE` privileges on shared system catalogs.

Unfortunately, there appears to be no race-proof way to serialize access
to `pg_shdescription` otherwise, because internal system catalog scans
do not obey current transaction isolation rules.  So, we do the next
best thing: poll `pg_locks` directly for a lock that looks like a
`COMMENT` lock on the template database.

edgedb/edgedb,edgedb/edgedb,edgedb/edgedb