[PROPOSAL] Drop orphan temp tables in single-mode

Hello hackers,

In some cases if PostgreSQL encounters with wraparound PostgreSQL might 
leave created temporary tables even after shutdown.

This orphan temporary tables prevent VACUUM to fix wraparound. It is 
because in single mode VACUUM considers orphan temp tables as temp 
tables of other backends.

Grigory reported that one of our client did stuck with fixing wraparound 
by because he didn't know that he has orphaned temp tables left by a 
backend after wraparound.

This patch fixes the issue. With it VACUUM deletes orphaned tables in 
single mode.

See also thread in general (I'm not sure that orphan temp tables were 
cause here though):
https://www.postgresql.org/message-id/CADU5SwN6u4radqQgUY2VjEyqXF0KJ6A09PYuJjT%3Do9d7vzM%3DCg%40mail.gmail.com

If the patch is interesting I'll add it to the next commitfest and label 
it as 'v13'.

-- 
Arthur Zakirov
Postgres Professional: http://www.postgrespro.com
Russian Postgres Company