pgsql: - Add check of already changed page while replay WAL.

Log Message:
-----------
- Add check of already changed page while replay WAL. This touches only
ginRedoInsert(), because other ginRedo* functions rewrite whole page or
make changes which could be applied several times without consistent's loss

- Remove check of identifying of corresponding split record:
it's possible that replaying of WAL starts after actual page split, but before
removing of that split from incomplete splits list. In this case, that check
cause FATAL error.

Per stress test which reproduces bug reported by Craig McElroy
<craig(dot)mcelroy(at)contegix(dot)com>

Tags:
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REL8_2_STABLE

Modified Files:
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pgsql/src/backend/access/gin:
ginxlog.c (r1.5.2.1 -> r1.5.2.2)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/gin/ginxlog.c?r1=1.5.2.1&r2=1.5.2.2)