pg_visibility: Add pg_truncate_visibility_map function.

Robert Haas <rhaas@postgresql.org>

Commit: 71d05a2c7b82379bb1013a0e338906349c54ed85
Author: Robert Haas <rhaas@postgresql.org>
Date: 2016-06-17T21:37:30Z
Releases: 9.6.0
pg_visibility: Add pg_truncate_visibility_map function.

This requires some core changes as well so that we can properly
WAL-log the truncation.  Specifically, it changes the format of the
XLOG_SMGR_TRUNCATE WAL record, so bump XLOG_PAGE_MAGIC.

Patch by me, reviewed but not fully endorsed by Andres Freund.

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