htlcswitch: extend Mailbox iface with dust, fee methods

This commit extends the Mailbox interface with the SetDustClosure,
SetFeeRate, and DustPackets methods. This enables the mailbox to
report the dust exposure to the Switch when the Switch decides whether
to forward a dust packet. The dust is counted from the time an Add is
introduced via AddPacket until it is removed via AckPacket. This can
lead to some packets being counted twice before they are signed for,
but this is a trade-off between accuracy and simplicity.

htlcswitch/link.go

@@ -12,6 +12,7 @@ import (
 
 	"github.com/btcsuite/btcd/wire"
 	"github.com/btcsuite/btclog"
+	"github.com/btcsuite/btcutil"
 	"github.com/davecgh/go-spew/spew"
 	"github.com/go-errors/errors"
 	"github.com/lightningnetwork/lnd/build"
@@ -2192,6 +2193,36 @@ func (l *channelLink) MayAddOutgoingHtlc() error {
 	return l.channel.MayAddOutgoingHtlc()
 }
 
+// dustClosure is a function that evaluates whether an HTLC is dust. It returns
+// true if the HTLC is dust. It takes in a feerate, a boolean denoting whether
+// the HTLC is incoming (i.e. one that the remote sent), a boolean denoting
+// whether to evaluate on the local or remote commit, and finally an HTLC
+// amount to test.
+type dustClosure func(chainfee.SatPerKWeight, bool, bool, btcutil.Amount) bool
+
+// dustHelper is used to construct the dustClosure.
+func dustHelper(chantype channeldb.ChannelType, localDustLimit,
+	remoteDustLimit btcutil.Amount) dustClosure {
+
+	isDust := func(feerate chainfee.SatPerKWeight, incoming,
+		localCommit bool, amt btcutil.Amount) bool {
+
+		if localCommit {
+			return lnwallet.HtlcIsDust(
+				chantype, incoming, true, feerate, amt,
+				localDustLimit,
+			)
+		}
+
+		return lnwallet.HtlcIsDust(
+			chantype, incoming, false, feerate, amt,
+			remoteDustLimit,
+		)
+	}
+
+	return isDust
+}
+
 // AttachMailBox updates the current mailbox used by this link, and hooks up
 // the mailbox's message and packet outboxes to the link's upstream and
 // downstream chans, respectively.