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_3_STABLE

Modified Files:
--------------
    pgsql-server/src/backend/access/transam:
        xact.c (r1.135.2.1 -> r1.135.2.2)

(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/access/transam/xact.c.diff?r1=1.135.2.1&r2=1.135.2.2)
        xlog.c (r1.109.2.3 -> r1.109.2.4)

(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/access/transam/xlog.c.diff?r1=1.109.2.3&r2=1.109.2.4)
    pgsql-server/src/include/storage:
        lwlock.h (r1.6 -> r1.6.2.1)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/include/storage/lwlock.h.diff?r1=1.6&r2=1.6.2.1)