pageinspect: Fix relcache leak in gist_page_items().
The gist_page_items() function opened the index relation on first call and
closed it on the last call. But there's no guarantee that the function is
run to completion, leading to a relcache leak and warning at the end of
the transaction. To fix, refactor the function to return all the rows in
one call, as a tuplestore.
Reported-by: Tom Lane
Discussion: https://www.postgresql.org/message-id/234863.
1610916631%40sss.pgh.pa.us