lnwallet: adopt simple fee structure for commitment transactions

This commit modifies the prior funding workflow to account for fees
when creating the funding output. As a stop gap, the current fee for
the commitment transaction is now hard-coded at 5k satoshis. Once the
fee models are in place this should instead be some high multiple of
the current “average” fee rate within the network, continuing, the
proper fee should be adjusted from the commitment transaction has
outputs are added/removed.

lnwallet/reservation.go

@@ -141,10 +141,16 @@ func NewChannelReservation(capacity, fundingAmt btcutil.Amount, minFeeRate btcut
 
 	if fundingAmt == 0 {
 		ourBalance = 0
-		theirBalance = capacity
+		theirBalance = capacity - commitFee
 	} else {
-		ourBalance = fundingAmt
-		theirBalance = capacity - fundingAmt
+		// TODO(roasbeef): need to rework fee structure in general and
+		// also when we "unlock" dual funder within the daemon
+		if capacity == fundingAmt+commitFee { // Single funder
+			ourBalance = capacity - commitFee
+		} else {
+			ourBalance = fundingAmt - commitFee
+		}
+		theirBalance = capacity - fundingAmt - commitFee
 	}
 
 	return &ChannelReservation{