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.

Modified Files:
--------------
    pgsql-server/src/backend/access/transam:
        xact.c (r1.177 -> r1.178)

(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/access/transam/xact.c.diff?r1=1.177&r2=1.178)
        xlog.c (r1.158 -> r1.159)

(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/access/transam/xlog.c.diff?r1=1.158&r2=1.159)
    pgsql-server/src/include/storage:
        lwlock.h (r1.12 -> r1.13)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/include/storage/lwlock.h.diff?r1=1.12&r2=1.13)