Fix bogus completion tag usage in walsender

Since commit fd5942c18f (2012, 9.3-era), walsender has been sending
completion tags for certain replication commands twice -- and they're
not even consistent.  Apparently neither libpq nor JDBC have a problem
with it, but it's not kosher.  Fix by remove the EndCommand() call in
the common code path for them all, and inserting specific calls to
EndReplicationCommand() specifically in those places where it's needed.

EndReplicationCommand() is a new simple function to send the completion
tag for replication commands.  Do this instead of sending a generic
SELECT completion tag for them all, which was also pretty bogus (if
innocuous).  While at it, change StartReplication() to use
EndReplicationCommand() instead of pg_puttextmessage().

In commit 2f9661311b, I failed to realize that replication commands
are not close-enough kin of regular SQL commands, so the
DROP_REPLICATION_SLOT tag I added is undeserved and a type pun.  Take it
out.

Backpatch to 13, where the latter commit appeared.  The duplicate tag
has been sent since 9.3, but since nothing is broken, it doesn't seem
worth fixing.

Per complaints from Tom Lane.

Discussion: https://postgr.es/m/1347966.1600195735@sss.pgh.pa.us

src/backend/tcop/dest.c

@@ -211,6 +211,18 @@ EndCommand(const QueryCompletion *qc, CommandDest dest, bool force_undecorated_o
 	}
 }
 
+/* ----------------
+ *		EndReplicationCommand - stripped down version of EndCommand
+ *
+ *		For use by replication commands.
+ * ----------------
+ */
+void
+EndReplicationCommand(const char *commandTag)
+{
+	pq_putmessage('C', commandTag, strlen(commandTag) + 1);
+}
+
 /* ----------------
  *		NullCommand - tell dest that an empty query string was recognized
  *