lnwallet: use btcutil.NewAmount for BTC -> SAT conversions

In this commit, we fix an existing rounding related bug in the codebase.
The RPC interface for btcd and bitcoind return values in BTC rather than
in satoshis. So in several places, we're forced to convert ourselves
manually. The existing logic attempted to do this, but didn't properly
account for rounding. As a result, our values can be off due to not
rounding incorrectly.

The fix for this is easy: simply properly use btcutil.NewAmount
everywhere which does rounding properly.

Fixes #939.

lnwallet/channel_test.go

@@ -163,7 +163,11 @@ func forceStateTransition(chanA, chanB *LightningChannel) error {
 func createTestChannels(revocationWindow int) (*LightningChannel,
 	*LightningChannel, func(), error) {
 
-	channelCapacity := btcutil.Amount(10 * 1e8)
+	channelCapacity, err := btcutil.NewAmount(10)
+	if err != nil {
+		return nil, nil, nil, err
+	}
+
 	channelBal := channelCapacity / 2
 	aliceDustLimit := btcutil.Amount(200)
 	bobDustLimit := btcutil.Amount(1300)