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)