Fix the initialization of atomic variable introduced by the

Fix the initialization of atomic variable introduced by the
group clearing mechanism.

Commit 0e141c0fbb introduced initialization of atomic variable in
InitProcess which means that it's not safe to look at it for backends that
aren't currently in use.  Fix that by initializing the same during postmaster
startup.

Reported-by: Andres Freund
Author: Amit Kapila
Backpatch-through: 9.6
Discussion:https://postgr.es/m/20181027104138.qmbbelopvy7cw2qv@alap3.anarazel.de