pgsql: Refactor XLogOpenRelation() and XLogReadBuffer() in preparation

Log Message:
-----------
Refactor XLogOpenRelation() and XLogReadBuffer() in preparation for relation
forks. XLogOpenRelation() and the associated light-weight relation cache in
xlogutils.c is gone, and XLogReadBuffer() now takes a RelFileNode as argument,
instead of Relation.

For functions that still need a Relation struct during WAL replay, there's a
new function called CreateFakeRelcacheEntry() that returns a fake entry like
XLogOpenRelation() used to.

Modified Files:
--------------
    pgsql/src/backend/access/gin:
        ginxlog.c (r1.13 -> r1.14)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/gin/ginxlog.c?r1=1.13&r2=1.14)
    pgsql/src/backend/access/gist:
        gist.c (r1.150 -> r1.151)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/gist/gist.c?r1=1.150&r2=1.151)
        gistutil.c (r1.26 -> r1.27)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/gist/gistutil.c?r1=1.26&r2=1.27)
        gistvacuum.c (r1.35 -> r1.36)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/gist/gistvacuum.c?r1=1.35&r2=1.36)
        gistxlog.c (r1.28 -> r1.29)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/gist/gistxlog.c?r1=1.28&r2=1.29)
    pgsql/src/backend/access/heap:
        heapam.c (r1.258 -> r1.259)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/heap/heapam.c?r1=1.258&r2=1.259)
        pruneheap.c (r1.13 -> r1.14)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/heap/pruneheap.c?r1=1.13&r2=1.14)
    pgsql/src/backend/access/nbtree:
        nbtxlog.c (r1.51 -> r1.52)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/nbtree/nbtxlog.c?r1=1.51&r2=1.52)
    pgsql/src/backend/access/transam:
        xlog.c (r1.313 -> r1.314)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xlog.c?r1=1.313&r2=1.314)
        xlogutils.c (r1.54 -> r1.55)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xlogutils.c?r1=1.54&r2=1.55)
    pgsql/src/backend/commands:
        sequence.c (r1.152 -> r1.153)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/sequence.c?r1=1.152&r2=1.153)
    pgsql/src/backend/storage/buffer:
        bufmgr.c (r1.231 -> r1.232)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/buffer/bufmgr.c?r1=1.231&r2=1.232)
        localbuf.c (r1.79 -> r1.80)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/buffer/localbuf.c?r1=1.79&r2=1.80)
    pgsql/src/backend/storage/smgr:
        smgr.c (r1.109 -> r1.110)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/smgr/smgr.c?r1=1.109&r2=1.110)
    pgsql/src/backend/utils/init:
        flatfiles.c (r1.34 -> r1.35)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/init/flatfiles.c?r1=1.34&r2=1.35)
    pgsql/src/include/access:
        gist_private.h (r1.29 -> r1.30)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/access/gist_private.h?r1=1.29&r2=1.30)
        heapam.h (r1.135 -> r1.136)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/access/heapam.h?r1=1.135&r2=1.136)
        xlogutils.h (r1.23 -> r1.24)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/access/xlogutils.h?r1=1.23&r2=1.24)
    pgsql/src/include/storage:
        buf_internals.h (r1.95 -> r1.96)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/storage/buf_internals.h?r1=1.95&r2=1.96)
        bufmgr.h (r1.112 -> r1.113)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/storage/bufmgr.h?r1=1.112&r2=1.113)