Consistency improvements for slot and decoding code.

Andres Freund <andres@anarazel.de>

Commit: e04a9ccd2ccd6e31cc4af6b08257a0a186d0fce8
Author: Andres Freund <andres@anarazel.de>
Date: 2014-06-12T11:33:27Z
Releases: 9.5.0
Consistency improvements for slot and decoding code.

Change the order of checks in similar functions to be the same; remove
a parameter that's not needed anymore; rename a memory context and
expand a couple of comments.

Per review comments from Amit Kapila

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