XLogReader general code cleanup

Alvaro Herrera <alvherre@alvh.no-ip.org>

Commit: 3b02ea4f0780ccce7dc116010201dad7ee50a401
Author: Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: 2016-03-30T21:56:13Z
Releases: 9.6.0
XLogReader general code cleanup

Some minor tweaks and comment additions, for cleanliness sake and to
avoid having the upcoming timeline-following patch be polluted with
unrelated cleanup.

Extracted from a larger patch by Craig Ringer, reviewed by Andres
Freund, with some additions by myself.

Files