Arrange to cache btree metapage data in the relcache entry for the index,

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.