Wire up query cancel interrupt for walsender backends.

This allows to cancel commands run over replication connections. While
it might have some use before v10, it has become important now that
normal SQL commands are allowed in database connected walsender
connections.

Author: Petr Jelinek
Reviewed-By: Andres Freund, Michael Paquier
Discussion: https://postgr.es/m/7966f454-7cd7-2b0c-8b70-cdca9d5a8c97@2ndquadrant.com

src/backend/replication/walsender.c

@@ -2929,7 +2929,7 @@ WalSndSignals(void)
 	/* Set up signal handlers */
 	pqsignal(SIGHUP, PostgresSigHupHandler);	/* set flag to read config
 												 * file */
-	pqsignal(SIGINT, SIG_IGN);	/* not used */
+	pqsignal(SIGINT, StatementCancelHandler);	/* query cancel */
 	pqsignal(SIGTERM, die);		/* request shutdown */
 	pqsignal(SIGQUIT, quickdie);	/* hard crash time */
 	InitializeTimeouts();		/* establishes SIGALRM handler */