multi: wait for rpcclients shutdown to complete

We need to call `WaitForShutdown` after stopping the rpc clients of the
chain backends.

chainntnfs/bitcoindnotify/bitcoind.go

@@ -139,6 +139,7 @@ func (b *BitcoindNotifier) Stop() error {
 	// Shutdown the rpc client, this gracefully disconnects from bitcoind,
 	// and cleans up all related resources.
 	b.chainConn.Stop()
+	b.chainConn.WaitForShutdown()
 
 	close(b.quit)
 	b.wg.Wait()