Validate page level checksums in base backups

Validate page level checksums in base backups

When base backups are run over the replication protocol (for example
using pg_basebackup), verify the checksums of all data blocks if
checksums are enabled. If checksum failures are encountered, log them
as warnings but don't abort the backup.

This becomes the default behaviour in pg_basebackup (provided checksums
are enabled on the server), so add a switch (-k) to disable the checks
if necessary.

Author: Michael Banck
Reviewed-By: Magnus Hagander, David Steele
Discussion: https://postgr.es/m/20180228180856.GE13784@nighthawk.caipicrew.dd-dns.de