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/reservation.go

@@ -110,6 +110,10 @@ type ChannelReservation struct {
 	// throughout its lifetime.
 	reservationID uint64
 
+	// pendingChanID is the pending channel ID for this channel as
+	// identified within the wire protocol.
+	pendingChanID [32]byte
+
 	// pushMSat the amount of milli-satoshis that should be pushed to the
 	// responder of a single funding channel as part of the initial
 	// commitment state.
@@ -129,7 +133,8 @@ func NewChannelReservation(capacity, localFundingAmt btcutil.Amount,
 	commitFeePerKw chainfee.SatPerKWeight, wallet *LightningWallet,
 	id uint64, pushMSat lnwire.MilliSatoshi, chainHash *chainhash.Hash,
 	flags lnwire.FundingFlag, tweaklessCommit bool,
-	fundingAssembler chanfunding.Assembler) (*ChannelReservation, error) {
+	fundingAssembler chanfunding.Assembler,
+	pendingChanID [32]byte) (*ChannelReservation, error) {
 
 	var (
 		ourBalance   lnwire.MilliSatoshi
@@ -263,6 +268,7 @@ func NewChannelReservation(capacity, localFundingAmt btcutil.Amount,
 			Db: wallet.Cfg.Database,
 		},
 		pushMSat:      pushMSat,
+		pendingChanID: pendingChanID,
 		reservationID: id,
 		wallet:        wallet,
 		chanFunder:    fundingAssembler,