pgsql: Arrange to cache btree metapage data in the relcache entry for

Log Message:
-----------
Arrange to cache btree metapage data in the relcache entry for the index,
thereby saving a visit to the metapage in most index searches/updates.
This wouldn't actually save any I/O (since in the old regime the metapage
generally stayed in cache anyway), but it does provide a useful decrease
in bufmgr traffic in high-contention scenarios.  Per my recent proposal.

Modified Files:
--------------
    pgsql/src/backend/access/nbtree:
        README (r1.9 -> r1.10)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/nbtree/README.diff?r1=1.9&r2=1.10)
        nbtinsert.c (r1.135 -> r1.136)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/nbtree/nbtinsert.c.diff?r1=1.135&r2=1.136)
        nbtpage.c (r1.95 -> r1.96)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/nbtree/nbtpage.c.diff?r1=1.95&r2=1.96)
        nbtree.c (r1.144 -> r1.145)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/nbtree/nbtree.c.diff?r1=1.144&r2=1.145)
    pgsql/src/backend/utils/cache:
        relcache.c (r1.238 -> r1.239)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/cache/relcache.c.diff?r1=1.238&r2=1.239)
    pgsql/src/include/utils:
        rel.h (r1.88 -> r1.89)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/rel.h.diff?r1=1.88&r2=1.89)