CREATE INDEX: don't update table stats if autovacuum=off.

CREATE INDEX: don't update table stats if autovacuum=off.

We previously fixed this for binary upgrade in 71b66171d0, but a
similar problem remained when dumping statistics without data.

Fix by not opportunistically updating table stats during CREATE INDEX
when autovacuum is disabled. For stats to be stable at all, the server
needs to be aware that it should not take every opportunity to update
stats. Per discussion, autovacuum=off is a signal that the user
expects stats to be stable; though if necessary, we could create
a more specific mode in the future.

Reported-by: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Discussion: https://postgr.es/m/CAExHW5vf9D+8-a5_BEX3y=2y_xY9hiCxV1=C+FnxDvfprWvkng@mail.gmail.com
Discussion: https://postgr.es/m/ca81cbf6e6ea2af838df972801ad4da52640a503.camel%40j-davis.com