pgsql-server: Rearrange pg_subtrans handling as per recent discussion.

Log Message:
-----------
Rearrange pg_subtrans handling as per recent discussion.  pg_subtrans
updates are no longer WAL-logged nor even fsync'd; we do not need to,
since after a crash no old pg_subtrans data is needed again.  We truncate
pg_subtrans to RecentGlobalXmin at each checkpoint.  slru.c's API is
refactored a little bit to separate out the necessary decisions.

Modified Files:
--------------
    pgsql-server/src/backend/access/transam:
        clog.c (r1.22 -> r1.23)

(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/access/transam/clog.c.diff?r1=1.22&r2=1.23)
        rmgr.c (r1.14 -> r1.15)

(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/access/transam/rmgr.c.diff?r1=1.14&r2=1.15)
        slru.c (r1.18 -> r1.19)

(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/access/transam/slru.c.diff?r1=1.18&r2=1.19)
        subtrans.c (r1.2 -> r1.3)

(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/access/transam/subtrans.c.diff?r1=1.2&r2=1.3)
        xlog.c (r1.162 -> r1.163)

(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/access/transam/xlog.c.diff?r1=1.162&r2=1.163)
    pgsql-server/src/backend/commands:
        vacuum.c (r1.286 -> r1.287)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/commands/vacuum.c.diff?r1=1.286&r2=1.287)
    pgsql-server/src/backend/storage/ipc:
        sinval.c (r1.69 -> r1.70)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/storage/ipc/sinval.c.diff?r1=1.69&r2=1.70)
    pgsql-server/src/backend/storage/lmgr:
        lwlock.c (r1.21 -> r1.22)

(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/storage/lmgr/lwlock.c.diff?r1=1.21&r2=1.22)
    pgsql-server/src/include/access:
        clog.h (r1.9 -> r1.10)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/include/access/clog.h.diff?r1=1.9&r2=1.10)
        rmgr.h (r1.11 -> r1.12)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/include/access/rmgr.h.diff?r1=1.11&r2=1.12)
        slru.h (r1.7 -> r1.8)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/include/access/slru.h.diff?r1=1.7&r2=1.8)
        subtrans.h (r1.2 -> r1.3)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/include/access/subtrans.h.diff?r1=1.2&r2=1.3)
    pgsql-server/src/include/storage:
        lwlock.h (r1.13 -> r1.14)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/include/storage/lwlock.h.diff?r1=1.13&r2=1.14)