This will allow the pending patch for parallel CREATE INDEX to work
on system catalogs, and to provide the same level of protection
against use of user indexes while they are being rebuilt that we
have for non-parallel CREATE INDEX.

Patch by me, reviewed by Peter Geoghegan.

Discussion: http://postgr.es/m/CA+TgmoYN-YQU9JsGQcqFLovZ-C+Xgp1_xhJQad=cunGG-_p5gg@mail.gmail.com
Discussion: http://postgr.es/m/CAH2-Wzkv4UNkXYhqQRqk-u9rS7h5c-4cCW+EqQ8K_WSeS43aZg@mail.gmail.com