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 <[email protected]>
Co-authored-by: Fabrízio de Royes Mello <[email protected]>
Reviewed-by: Jacob Champion <[email protected]>
Reviewed-by: Álvaro Herrera <[email protected]>
Reviewed-by: Euler Taveira <[email protected]>
Discussion: https://postgr.es/m/aIECfYfevCUpenBT@nathan
Discussion: https://postgr.es/m/CAFcNs%2Br73NOUb7%2BqKrV4HHEki02CS96Z%2Bx19WaFgE087BWwEng%40mail.gmail.com