htlcswitch+lnwallet: calculate fee exposure as commit fees + dust

This commit expands the definition of the dust limit to take into
account commitment fees as well as dust HTLCs. The dust limit is now
known as a fee exposure threshold. Dust HTLCs are fees anyways so it
makes sense to account for commitment fees as well. The link has
been modified slightly to calculate dust. In the future, the switch
dust calculations can be removed.

config.go

@@ -441,7 +441,7 @@ type Config struct {
 
 	GcCanceledInvoicesOnTheFly bool `long:"gc-canceled-invoices-on-the-fly" description:"If true, we'll delete newly canceled invoices on the fly."`
 
-	DustThreshold uint64 `long:"dust-threshold" description:"Sets the dust sum threshold in satoshis for a channel after which dust HTLC's will be failed."`
+	MaxFeeExposure uint64 `long:"dust-threshold" description:"Sets the max fee exposure in satoshis for a channel after which HTLC's will be failed."`
 
 	Fee *lncfg.Fee `group:"fee" namespace:"fee"`
 
@@ -684,7 +684,7 @@ func DefaultConfig() Config {
 		MaxOutgoingCltvExpiry:     htlcswitch.DefaultMaxOutgoingCltvExpiry,
 		MaxChannelFeeAllocation:   htlcswitch.DefaultMaxLinkFeeAllocation,
 		MaxCommitFeeRateAnchors:   lnwallet.DefaultAnchorsCommitMaxFeeRateSatPerVByte,
-		DustThreshold:             uint64(htlcswitch.DefaultDustThreshold.ToSatoshis()),
+		MaxFeeExposure:            uint64(htlcswitch.DefaultMaxFeeExposure.ToSatoshis()),
 		LogWriter:                 build.NewRotatingLogWriter(),
 		DB:                        lncfg.DefaultDB(),
 		Cluster:                   lncfg.DefaultCluster(),