Rename PGPROC fields related to group XID clearing again.

Rename PGPROC fields related to group XID clearing again.

Commit 0e141c0fbb211bdd23783afa731e3eef95c9ad7a introduced a new
facility to reduce ProcArrayLock contention by clearing several XIDs
from the ProcArray under a single lock acquisition.  The names
initially chosen were deemed not to be very good choices, so commit
4aec49899e5782247e134f94ce1c6ee926f88e1c renamed them.  But now it
seems like we still didn't get it right.  A pending patch wants to
add similar infrastructure for batching CLOG updates, so the names
need to be clear enough to allow a new set of structure members with
a related purpose.

Amit Kapila