multi: obtain+verify aux sigs for all second level HTLCs

In this commit, we start to use the new AuxSigner to obtain+verify aux sigs for all second level HTLCs. This is similar to the existing SigPool, but we'll only attempt to do this if the AuxSigner is present (won't be for most channels).

lnwallet/wallet.go

@@ -2607,6 +2607,9 @@ func (l *LightningWallet) ValidateChannel(channelState *channeldb.OpenChannel,
 	l.Cfg.AuxLeafStore.WhenSome(func(s AuxLeafStore) {
 		chanOpts = append(chanOpts, WithLeafStore(s))
 	})
+	l.Cfg.AuxSigner.WhenSome(func(s AuxSigner) {
+		chanOpts = append(chanOpts, WithAuxSigner(s))
+	})
 
 	// First, we'll obtain a fully signed commitment transaction so we can
 	// pass into it on the chanvalidate package for verification.