pgsql: Clean up representation of SLRU page state.

Log Message:
-----------
Clean up representation of SLRU page state.  This is the cleaner fix
for the SLRU race condition that I posted a few days ago, but we decided
not to use in 8.1 and older branches.

Modified Files:
--------------
    pgsql/src/backend/access/transam:
        clog.c (r1.33 -> r1.34)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/clog.c.diff?r1=1.33&r2=1.34)
        multixact.c (r1.11 -> r1.12)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/multixact.c.diff?r1=1.11&r2=1.12)
        slru.c (r1.29 -> r1.30)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/slru.c.diff?r1=1.29&r2=1.30)
        subtrans.c (r1.11 -> r1.12)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/subtrans.c.diff?r1=1.11&r2=1.12)
    pgsql/src/include/access:
        slru.h (r1.14 -> r1.15)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/slru.h.diff?r1=1.14&r2=1.15)