lnwallet+funding: pass the pending channel ID into the reservation context

In this commit, we start to thread the pending channel ID from wire
protocol all the way down into the reservation context. This change will
allow negotiation to take place _outside_ the protocol that may result
in a particular chanfunding.Assembler being dispatched.

lnwallet/wallet.go

@@ -48,6 +48,10 @@ type InitFundingReserveMsg struct {
 	// target channel.
 	ChainHash *chainhash.Hash
 
+	// PendingChanID is the pending channel ID for this funding flow as
+	// used in the wire protocol.
+	PendingChanID [32]byte
+
 	// NodeID is the ID of the remote node we would like to open a channel
 	// with.
 	NodeID *btcec.PublicKey
@@ -513,7 +517,7 @@ func (l *LightningWallet) handleFundingReserveRequest(req *InitFundingReserveMsg
 	reservation, err := NewChannelReservation(
 		capacity, localFundingAmt, req.CommitFeePerKw, l, id,
 		req.PushMSat, l.Cfg.NetParams.GenesisHash, req.Flags,
-		req.Tweakless, req.ChanFunder,
+		req.Tweakless, req.ChanFunder, req.PendingChanID,
 	)
 	if err != nil {
 		if fundingIntent != nil {