Remove secondary checkpoint

Simon Riggs <simon@2ndQuadrant.com>

Commit: 4b0d28de06b28e57c540fca458e4853854fbeaf8
Author: Simon Riggs <simon@2ndQuadrant.com>
Date: 2017-11-07T17:56:30Z
Releases: 11.0
Remove secondary checkpoint

Previously server reserved WAL for last two checkpoints,
which used too much disk space for small servers.

Bumps PG_CONTROL_VERSION

Author: Simon Riggs <simon@2ndQuadrant.com>
Reviewed-by: Michael Paquier <michael.paquier@gmail.com>

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Documentation touched