pgsql-server: Fix failure to guarantee that a checkpoint will write out

Log Message:
-----------
Fix failure to guarantee that a checkpoint will write out pg_clog updates
for transaction commits that occurred just before the checkpoint. This is
an EXTREMELY serious bug --- kudos to Satoshi Okada for creating a
reproducible test case to prove its existence.

Tags:
----
REL7_2_STABLE

Modified Files:
--------------
pgsql-server/src/backend/access/transam:
xact.c (r1.115.2.1 -> r1.115.2.2)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/access/transam/xact.c.diff?r1=1.115.2.1&r2=1.115.2.2)
xlog.c (r1.86.2.3 -> r1.86.2.4)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/access/transam/xlog.c.diff?r1=1.86.2.3&r2=1.86.2.4)
pgsql-server/src/include/storage:
lwlock.h (r1.4 -> r1.4.2.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/include/storage/lwlock.h.diff?r1=1.4&r2=1.4.2.1)