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_4_STABLE

Modified Files:
--------------
    pgsql-server/src/backend/access/transam:
        xact.c (r1.156 -> r1.156.2.1)

(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/access/transam/xact.c.diff?r1=1.156&r2=1.156.2.1)
        xlog.c (r1.125.2.1 -> r1.125.2.2)

(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/access/transam/xlog.c.diff?r1=1.125.2.1&r2=1.125.2.2)
    pgsql-server/src/include/storage:
        lwlock.h (r1.8 -> r1.8.4.1)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/include/storage/lwlock.h.diff?r1=1.8&r2=1.8.4.1)