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.

server.go

@@ -638,7 +638,7 @@ func newServer(cfg *Config, listenAddrs []net.Addr,
 
 	s.htlcNotifier = htlcswitch.NewHtlcNotifier(time.Now)
 
-	thresholdSats := btcutil.Amount(cfg.DustThreshold)
+	thresholdSats := btcutil.Amount(cfg.MaxFeeExposure)
 	thresholdMSats := lnwire.NewMSatFromSatoshis(thresholdSats)
 
 	s.aliasMgr, err = aliasmgr.NewManager(dbs.ChanStateDB)
@@ -678,7 +678,7 @@ func newServer(cfg *Config, listenAddrs []net.Addr,
 		RejectHTLC:             cfg.RejectHTLC,
 		Clock:                  clock.NewDefaultClock(),
 		MailboxDeliveryTimeout: cfg.Htlcswitch.MailboxDeliveryTimeout,
-		DustThreshold:          thresholdMSats,
+		MaxFeeExposure:         thresholdMSats,
 		SignAliasUpdate:        s.signAliasUpdate,
 		IsAlias:                aliasmgr.IsAlias,
 	}, uint32(currentHeight))
@@ -3863,6 +3863,9 @@ func (s *server) peerConnected(conn net.Conn, connReq *connmgr.ConnReq,
 		towerClient = s.towerClientMgr
 	}
 
+	thresholdSats := btcutil.Amount(s.cfg.MaxFeeExposure)
+	thresholdMSats := lnwire.NewMSatFromSatoshis(thresholdSats)
+
 	// Now that we've established a connection, create a peer, and it to the
 	// set of currently active peers. Configure the peer with the incoming
 	// and outgoing broadcast deltas to prevent htlcs from being accepted or
@@ -3932,6 +3935,7 @@ func (s *server) peerConnected(conn net.Conn, connReq *connmgr.ConnReq,
 		RequestAlias:           s.aliasMgr.RequestAlias,
 		AddLocalAlias:          s.aliasMgr.AddLocalAlias,
 		DisallowRouteBlinding:  s.cfg.ProtocolOptions.NoRouteBlinding(),
+		MaxFeeExposure:         thresholdMSats,
 		Quit:                   s.quit,
 	}