lnwallet: update channel state machine to use new ScriptDescriptor interface

In this commit, we update the channel state machine to use the new
ScriptDescriptor interface. This fixes some subtle issues with the
existing commits, as for p2wsh we always sign the same witness script,
but for p2tr, the witness script differs depending on which branch is
taken.

With the new abstractions, we can treat p2wsh and p2tr as the same
mostly, right up until we need to obtain a control block or a tap tweak.

All tests have been updated accordingly.

lnwallet/transactions.go

@@ -66,8 +66,6 @@ func CreateHtlcSuccessTx(chanType channeldb.ChannelType, initiator bool,
 	}
 	successTx.AddTxIn(txin)
 
-	var pkScript []byte
-
 	// Next, we'll generate the script used as the output for all second
 	// level HTLC which forces a covenant w.r.t what can be done with all
 	// HTLC outputs.
@@ -79,13 +77,11 @@ func CreateHtlcSuccessTx(chanType channeldb.ChannelType, initiator bool,
 		return nil, err
 	}
 
-	pkScript = scriptInfo.PkScript
-
 	// Finally, the output is simply the amount of the HTLC (minus the
 	// required fees), paying to the timeout script.
 	successTx.AddTxOut(&wire.TxOut{
 		Value:    int64(htlcAmt),
-		PkScript: pkScript,
+		PkScript: scriptInfo.PkScript(),
 	})
 
 	return successTx, nil
@@ -133,8 +129,6 @@ func CreateHtlcTimeoutTx(chanType channeldb.ChannelType, initiator bool,
 	}
 	timeoutTx.AddTxIn(txin)
 
-	var pkScript []byte
-
 	// Next, we'll generate the script used as the output for all second
 	// level HTLC which forces a covenant w.r.t what can be done with all
 	// HTLC outputs.
@@ -146,13 +140,11 @@ func CreateHtlcTimeoutTx(chanType channeldb.ChannelType, initiator bool,
 		return nil, err
 	}
 
-	pkScript = scriptInfo.PkScript
-
 	// Finally, the output is simply the amount of the HTLC (minus the
 	// required fees), paying to the regular second level HTLC script.
 	timeoutTx.AddTxOut(&wire.TxOut{
 		Value:    int64(htlcAmt),
-		PkScript: pkScript,
+		PkScript: scriptInfo.PkScript(),
 	})
 
 	return timeoutTx, nil