Use PqMsg_* macros in more places.

Commit f4b54e1ed9, which introduced macros for protocol characters,
missed updating a few places.  It also did not introduce macros for
messages sent from parallel workers to their leader processes.
This commit adds a new section in protocol.h for those.

Author: Aleksander Alekseev
Discussion: https://postgr.es/m/CAJ7c6TNTd09AZq8tGaHS3LDyH_CCnpv0oOz2wN1dGe8zekxrdQ%40mail.gmail.com
Backpatch-through: 17

src/backend/access/common/printtup.c

@@ -17,6 +17,7 @@
 
 #include "access/printtup.h"
 #include "libpq/pqformat.h"
+#include "libpq/protocol.h"
 #include "tcop/pquery.h"
 #include "utils/lsyscache.h"
 #include "utils/memdebug.h"
@@ -170,7 +171,7 @@ SendRowDescriptionMessage(StringInfo buf, TupleDesc typeinfo,
 	ListCell   *tlist_item = list_head(targetlist);
 
 	/* tuple descriptor message type */
-	pq_beginmessage_reuse(buf, 'T');
+	pq_beginmessage_reuse(buf, PqMsg_RowDescription);
 	/* # of attrs in tuples */
 	pq_sendint16(buf, natts);
 
@@ -322,7 +323,7 @@ printtup(TupleTableSlot *slot, DestReceiver *self)
 	/*
 	 * Prepare a DataRow message (note buffer is in per-query context)
 	 */
-	pq_beginmessage_reuse(buf, 'D');
+	pq_beginmessage_reuse(buf, PqMsg_DataRow);
 
 	pq_sendint16(buf, natts);