htlcswicth+channel: switch to store onion blobs in payment descriptor

After addition of the retransmission logic in the channel link, we
should make the onion blobs persistant, the proper way to do this is
include the onion blobs in the payment descriptor rather than storing
them in the distinct struct in the channel link.

lnwallet/channel.go

@@ -208,9 +208,11 @@ type PaymentDescriptor struct {
 	removeCommitHeightRemote uint64
 	removeCommitHeightLocal  uint64
 
-	// Payload is an opaque blob which is used to complete multi-hop
+	// OnionBlob is an opaque blob which is used to complete multi-hop
 	// routing.
-	Payload []byte
+	//
+	// NOTE: Populated only on add payment descriptor entry types.
+	OnionBlob []byte
 
 	// [our|their|]PkScript are the raw public key scripts that encodes the
 	// redemption rules for this particular HTLC. These fields will only be
@@ -534,6 +536,7 @@ func (c *commitment) toChannelDelta(ourCommit bool) (*channeldb.ChannelDelta, er
 			RHash:         htlc.RHash,
 			RefundTimeout: htlc.Timeout,
 			OutputIndex:   outputIndex,
+			OnionBlob:     htlc.OnionBlob,
 		}
 
 		if ourCommit && htlc.sig != nil {
@@ -1688,6 +1691,7 @@ func (lc *LightningChannel) restoreStateLogs() error {
 			EntryType:             Add,
 			addCommitHeightRemote: pastHeight,
 			addCommitHeightLocal:  pastHeight,
+			OnionBlob:             htlc.OnionBlob,
 			ourPkScript:           ourP2WSH,
 			ourWitnessScript:      ourWitnessScript,
 			theirPkScript:         theirP2WSH,
@@ -2487,7 +2491,7 @@ func (lc *LightningChannel) ReceiveReestablish(msg *lnwire.ChannelReestablish) (
 			switch htlc.EntryType {
 			case Add:
 				var onionBlob [lnwire.OnionPacketSize]byte
-				copy(onionBlob[:], htlc.Payload)
+				copy(onionBlob[:], htlc.OnionBlob)
 				updates = append(updates, &lnwire.UpdateAddHTLC{
 					ChanID:      chanID,
 					ID:          htlc.Index,
@@ -3102,6 +3106,7 @@ func (lc *LightningChannel) AddHTLC(htlc *lnwire.UpdateAddHTLC) (uint64, error)
 		Amount:    htlc.Amount,
 		LogIndex:  lc.localUpdateLog.logIndex,
 		HtlcIndex: lc.localUpdateLog.htlcCounter,
+		OnionBlob: htlc.OnionBlob[:],
 	}
 
 	lc.localUpdateLog.appendHtlc(pd)
@@ -3128,6 +3133,7 @@ func (lc *LightningChannel) ReceiveHTLC(htlc *lnwire.UpdateAddHTLC) (uint64, err
 		Amount:    htlc.Amount,
 		LogIndex:  lc.remoteUpdateLog.logIndex,
 		HtlcIndex: lc.remoteUpdateLog.htlcCounter,
+		OnionBlob: htlc.OnionBlob[:],
 	}
 
 	lc.remoteUpdateLog.appendHtlc(pd)