group clearing mechanism.

Commits 0e141c0f and baaf272a introduced initialization of atomic
variables in InitProcess which means that it's not safe to look at those
for backends that aren't currently in use.  Fix that by initializing them
during postmaster startup.

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