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:
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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)