pgsql: XLogReader general code cleanup

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.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/3b02ea4f0780ccce7dc116010201dad7ee50a401

Modified Files
--------------
src/backend/access/transam/xlogreader.c        | 54 ++++++++++++++++++--------
src/backend/access/transam/xlogutils.c         | 21 +++++++---
src/backend/replication/logical/logicalfuncs.c | 18 ++++++++-
src/include/access/xlogreader.h                | 17 ++++++--
src/include/access/xlogutils.h                 |  6 ++-
5 files changed, 86 insertions(+), 30 deletions(-)