lnwallet: track ChannelID on PaymentDescriptor

In this commit we track the ChannelID on the PaymentDescriptor.
This will be useful in upcoming commits that need to be able to
reconstruct lnwire.Message values from PaymentDescriptors as the
Messages that are exchanged to update channel state all include
the ChannelID.

lnwallet/payment_descriptor.go

@@ -70,6 +70,11 @@ func (u updateType) String() string {
 // TODO(roasbeef): LogEntry interface??
 //   - need to separate attrs for cancel/add/settle/feeupdate
 type PaymentDescriptor struct {
+	// ChanID is the ChannelID of the LightningChannel that this
+	// PaymentDescriptor belongs to. We track this here so we can
+	// reconstruct the Messages that this PaymentDescriptor is built from.
+	ChanID lnwire.ChannelID
+
 	// RHash is the payment hash for this HTLC. The HTLC can be settled iff
 	// the preimage to this hash is presented.
 	RHash PaymentHash