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
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REL9_4_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/d52a6c4944c1d83cfaae53e20561c53220e7968b

Modified Files
--------------
src/backend/access/transam/xlog.c         |    2 +-
src/backend/replication/logical/logical.c |    2 +-
src/backend/replication/slot.c            |    2 +-
src/backend/replication/slotfuncs.c       |   17 +++++++++++------
src/backend/replication/walsender.c       |    9 +++++++--
src/include/replication/slot.h            |    2 +-
6 files changed, 22 insertions(+), 12 deletions(-)