multi: wait for rpcclients shutdown to complete

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

go.mod

@@ -11,7 +11,7 @@ require (
 	github.com/btcsuite/btcd/chaincfg/chainhash v1.1.0
 	github.com/btcsuite/btclog v0.0.0-20241003133417-09c4e92e319c
 	github.com/btcsuite/btclog/v2 v2.0.0-20241017175713-3428138b75c7
-	github.com/btcsuite/btcwallet v0.16.10-0.20240912233857-ffb143c77cc5
+	github.com/btcsuite/btcwallet v0.16.10-0.20241113134707-b4ff60753aaa
 	github.com/btcsuite/btcwallet/wallet/txauthor v1.3.5
 	github.com/btcsuite/btcwallet/wallet/txrules v1.2.2
 	github.com/btcsuite/btcwallet/walletdb v1.4.4