Fix possible duplicate tuples while GiST scan. Now page is processed

Teodor Sigaev <teodor@sigaev.ru>

Commit: 1dcf6fdf1bd7fb2501445fa2ebfa57e04273e848
Author: Teodor Sigaev <teodor@sigaev.ru>
Date: 2008-08-23T10:37:24Z
Releases: 8.4.0
Fix possible duplicate tuples while  GiST scan. Now page is processed
at once and ItemPointers are collected in memory.

Remove tuple's killing by killtuple() if tuple was moved to another
page - it could produce unaceptable overhead.

Backpatch up to 8.1 because the bug was introduced by GiST's concurrency support.

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