Resetting PGPROC atomics in ProcessInit()

Hi,

I just noticed, while working on a patch adding things to PGPROC, that
the group clearning patches for the proc array and clog reset atomics in
InitProcess().

I'm not a big fan of that, because it means that it's not safe to look
at the atomics of backends that aren't currently in use.  Is there any
reason to not instead initialize them in InitProcGlobal() and just
assert in InitProcess() that they're 0?  If they're not, we'd be in deep
trouble anyway, no?

Greetings,

Andres Freund