add "future" open channel to reservation

p2p logic will most likely spawn a grouting to do a blocking read on
this channel. When the channel is written to, that means the funding
transaction is now X blocks deep, and we’ll start to sign commitment
txns do updates etc.

wallet/reservation.go

@@ -42,6 +42,8 @@ type ChannelReservation struct {
 
 	reservationID uint64
 	wallet        *LightningWallet
+
+	chanOpen chan *LightningChannel
 }
 
 // newChannelReservation...