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.

routing/router.go

@@ -137,7 +137,8 @@ type ChannelGraphSource interface {
 	// ForAllOutgoingChannels is used to iterate over all channels
 	// emanating from the "source" node which is the center of the
 	// star-graph.
-	ForAllOutgoingChannels(cb func(c *channeldb.ChannelEdgeInfo,
+	ForAllOutgoingChannels(cb func(tx kvdb.RTx,
+		c *channeldb.ChannelEdgeInfo,
 		e *channeldb.ChannelEdgePolicy) error) error
 
 	// CurrentBlockHeight returns the block height from POV of the router
@@ -2426,17 +2427,18 @@ func (r *ChannelRouter) ForEachNode(cb func(*channeldb.LightningNode) error) err
 // the router.
 //
 // NOTE: This method is part of the ChannelGraphSource interface.
-func (r *ChannelRouter) ForAllOutgoingChannels(cb func(*channeldb.ChannelEdgeInfo,
-	*channeldb.ChannelEdgePolicy) error) error {
+func (r *ChannelRouter) ForAllOutgoingChannels(cb func(kvdb.RTx,
+	*channeldb.ChannelEdgeInfo, *channeldb.ChannelEdgePolicy) error) error {
 
-	return r.selfNode.ForEachChannel(nil, func(_ kvdb.RTx, c *channeldb.ChannelEdgeInfo,
+	return r.selfNode.ForEachChannel(nil, func(tx kvdb.RTx,
+		c *channeldb.ChannelEdgeInfo,
 		e, _ *channeldb.ChannelEdgePolicy) error {
 
 		if e == nil {
 			return fmt.Errorf("channel from self node has no policy")
 		}
 
-		return cb(c, e)
+		return cb(tx, c, e)
 	})
 }