Avoid spurious wait in concurrent reindex

Avoid spurious wait in concurrent reindex

This is like commit c98763bf51bf, but for REINDEX CONCURRENTLY.  To wit:
this flags indicates that the current process is safe to ignore for the
purposes of waiting for other snapshots, when doing CREATE INDEX
CONCURRENTLY and REINDEX CONCURRENTLY.  This helps two processes doing
either of those things not deadlock, and also avoids spurious waits.

Author: Álvaro Herrera <[email protected]>
Reviewed-by: Dmitry Dolgov <[email protected]>
Reviewed-by: Hamid Akhtar <[email protected]>
Reviewed-by: Masahiko Sawada <[email protected]>
Discussion: https://postgr.es/m/20201130195439[email protected]