Don't attempt to verify checksums on new pages

Magnus Hagander <magnus@hagander.net>

Commit: 33cedf1474a356fe510d4ad32c040c968ef27307
Author: Magnus Hagander <magnus@hagander.net>
Date: 2018-04-15T12:05:56Z
Releases: 11.0
Don't attempt to verify checksums on new pages

Teach both base backups and pg_verify_checksums that if a page is new,
it does not have a checksum yet, so it shouldn't be verified.

Noted by Tomas Vondra, review by David Steele.

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