Reorganize logical replication worker disconnect code

Move the walrcv_disconnect() calls into the before_shmem_exit handler.
This makes sure the call is always made even during exit by signal, it
saves some duplicate code, and it makes the logic more similar to
walreceiver.c.

Author: Petr Jelinek <petr.jelinek@2ndquadrant.com>

src/backend/replication/logical/launcher.c

@@ -614,7 +614,13 @@ logicalrep_launcher_onexit(int code, Datum arg)
 static void
 logicalrep_worker_onexit(int code, Datum arg)
 {
+	/* Disconnect gracefully from the remote side. */
+	if (wrconn)
+		walrcv_disconnect(wrconn);
+
 	logicalrep_worker_detach();
+
+	ApplyLauncherWakeup();
 }
 
 /* SIGTERM: set flag to exit at next convenient time */