pgsql-server: Arrange to explicitly stop the pgstat processes at the

Log Message:
-----------
Arrange to explicitly stop the pgstat processes at the same time we
begin the shutdown checkpoint; there isn't anything left for them to do,
so we may as well ensure that they shut down sooner rather than later.
Per discussion.

Modified Files:
--------------
pgsql-server/src/backend/postmaster:
pgstat.c (r1.74 -> r1.75)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/postmaster/pgstat.c.diff?r1=1.74&r2=1.75)
postmaster.c (r1.403 -> r1.404)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/postmaster/postmaster.c.diff?r1=1.403&r2=1.404)
pgsql-server/src/include:
pgstat.h (r1.23 -> r1.24)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/include/pgstat.h.diff?r1=1.23&r2=1.24)