pgsql: Transfer state pertaining to pending REINDEX operations to worke

Transfer state pertaining to pending REINDEX operations to workers.

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

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/29d58fd3adae9057c3fd502393b2f131bc96eaf9

Modified Files
--------------
src/backend/access/transam/README.parallel | 3 ++
src/backend/access/transam/parallel.c | 18 ++++++-
src/backend/catalog/index.c | 75 +++++++++++++++++++++++++++++-
src/include/catalog/index.h | 4 ++
4 files changed, 98 insertions(+), 2 deletions(-)