Allow processes to reset procArrayGroupNext themselves instead of leader resetting for all the followers

Hi,

While working on something else, I noticed that the proc array group
XID clearing leader resets procArrayGroupNext of all the followers
atomically along with procArrayGroupMember. ISTM that it's enough for
the followers to exit the wait loop and continue if the leader resets
just procArrayGroupMember, the followers can reset procArrayGroupNext
by themselves. This relieves the leader a bit, especially when there
are many followers, as it avoids a bunch of atomic writes and
pg_write_barrier() for the leader .

I'm attaching a small patch with the above change. It doesn't seem to
break anything, the cirrus-ci members are happy
https://github.com/BRupireddy/postgres/tree/allow_processes_to_reset_proc_array_v1.
It doesn't seem to show visible benefit on my system, nor hurt anyone
in my testing [1].

Thoughts?

[1]
# of clients HEAD PATCHED
1 31661 31512
2 67134 66789
4 135084 132372
8 254174 255384
16 418598 420903
32 491922 494183
64 509824 509451
128 513298 512838
256 505191 496266
512 465208 453588
768 431304 425736
1024 398110 397352
2048 308732 308901
4096 200355 219480

--
Bharath Rupireddy
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com