wrapping CF 2021-09

Hi everyone,

At this time I'm looking to close the CF.

So, I will write and mark as RwF the patches currently failing on the
cfbot and move to the next CF all other patches.

RwF:
- ALTER SYSTEM READ { ONLY | WRITE }
- GUC to disable cancellation of awaiting for synchronous replication
- Introduce ProcessInterrupts_hook for C extensions
- Make message at end-of-recovery less scary

-- 
Jaime Casanova
Director de Servicios Profesionales
SystemGuards - Consultores de PostgreSQL