This is majorly backport of Postgres commit.
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commit a507b869
Author: Robert Haas <rhaas@postgresql.org>
Date:   Wed Feb 8 15:45:30 2017 -0500

    Add WAL consistency checking facility.

    When the new GUC wal_consistency_checking is set to a non-empty value,
    it triggers recording of additional full-page images, which are
    compared on the standby against the results of applying the WAL record
    (without regard to those full-page images).  Allowable differences
    such as hints are masked out, and the resulting pages are compared;
    any difference results in a FATAL error on the standby.

    Kuntal Ghosh, based on earlier patches by Michael Paquier and Heikki
    Linnakangas.  Extensively reviewed and revised by Michael Paquier and
    by me, with additional reviews and comments from Amit Kapila, Álvaro
    Herrera, Simon Riggs, and Peter Eisentraut.
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Its modified to work with current xlog format of Greenplum, which is different
from Postgres code when this patch was committed. Main changes are to fit
current backup block format in xlog records. Also, some masking routines differ
compared to upstream as some of the masked flags would come once Greenplum
catches up to latest versions of Postgres.