[COMMITTERS] pgsql: Consistency improvements for slot and decoding code.

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 Branch -- master Details --- h

[COMMITTERS] pgsql: Consistency improvements for slot and decoding code.

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 Branch -- REL9_4_STABLE Details --