pgsql: Expand usage of macros for protocol characters.

Expand usage of macros for protocol characters.

This commit makes use of the existing PqMsg_* macros in more places
and adds new PqReplMsg_* and PqBackupMsg_* macros for use in
special replication and backup messages, respectively.

Author: Dave Cramer <davecramer(at)gmail(dot)com>
Co-authored-by: Fabrízio de Royes Mello <fabriziomello(at)gmail(dot)com>
Reviewed-by: Jacob Champion <jacob(dot)champion(at)enterprisedb(dot)com>
Reviewed-by: Álvaro Herrera <alvherre(at)kurilemu(dot)de>
Reviewed-by: Euler Taveira <euler(at)eulerto(dot)com>
Discussion: https://postgr.es/m/aIECfYfevCUpenBT@nathan
Discussion: https://postgr.es/m/CAFcNs%2Br73NOUb7%2BqKrV4HHEki02CS96Z%2Bx19WaFgE087BWwEng%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/9ea3b6f751abd7701f3f32d9df26d66410754c94

Modified Files
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src/backend/backup/basebackup_copy.c | 14 ++++++------
.../replication/logical/applyparallelworker.c | 4 ++--
src/backend/replication/logical/worker.c | 10 ++++-----
src/backend/replication/walreceiver.c | 8 +++----
src/backend/replication/walsender.c | 25 ++++++++++++----------
src/bin/pg_basebackup/pg_basebackup.c | 9 ++++----
src/bin/pg_basebackup/pg_recvlogical.c | 11 +++++-----
src/bin/pg_basebackup/receivelog.c | 11 +++++-----
src/include/libpq/protocol.h | 21 ++++++++++++++++++
9 files changed, 70 insertions(+), 43 deletions(-)