pgsql: pgstat's on-proc-exit hook has to execute after the last

Log Message:
-----------
pgstat's on-proc-exit hook has to execute after the last transaction commit
or abort within a backend; rearrange InitPostgres processing to make it so.
Revealed by just-added Asserts along with ECPG regression tests (hm, I wonder
why the core regression tests didn't expose it?). This possibly is another
reason for missing stats updates ...

Modified Files:
--------------
pgsql/src/backend/postmaster:
pgstat.c (r1.156 -> r1.157)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/pgstat.c.diff?r1=1.156&r2=1.157)
pgsql/src/backend/utils/init:
postinit.c (r1.175 -> r1.176)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/init/postinit.c.diff?r1=1.175&r2=1.176)
pgsql/src/include:
pgstat.h (r1.59 -> r1.60)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/pgstat.h.diff?r1=1.59&r2=1.60)