channeldb+routing: add tx parameter

Adds an optional tx parameter to ForAllOutgoingChannels and FetchChannel
so that data can be queried within the context of an existing database
transaction.

discovery/gossiper.go

@@ -15,6 +15,7 @@ import (
 	"github.com/lightningnetwork/lnd/batch"
 	"github.com/lightningnetwork/lnd/chainntnfs"
 	"github.com/lightningnetwork/lnd/channeldb"
+	"github.com/lightningnetwork/lnd/kvdb"
 	"github.com/lightningnetwork/lnd/lnpeer"
 	"github.com/lightningnetwork/lnd/lnwallet"
 	"github.com/lightningnetwork/lnd/lnwire"
@@ -1210,6 +1211,7 @@ func (d *AuthenticatedGossiper) retransmitStaleAnns(now time.Time) error {
 		edgesToUpdate      []updateTuple
 	)
 	err := d.cfg.Router.ForAllOutgoingChannels(func(
+		_ kvdb.RTx,
 		info *channeldb.ChannelEdgeInfo,
 		edge *channeldb.ChannelEdgePolicy) error {