multi: use single read transaction for path finding

This commit partially reverts bf27d05a.
To avoid creating multiple database transactions during a single path
finding operation, we create an explicit transaction when the cached
graph is instantiated.
We cache the source node to avoid needing to look that up for every path
finding session.
The database transaction will be nil in case of the in-memory graph.

channeldb/graph.go

@@ -308,6 +308,16 @@ func initChannelGraph(db kvdb.Backend) error {
 	return nil
 }
 
+// NewPathFindTx returns a new read transaction that can be used for a single
+// path finding session. Will return nil if the graph cache is enabled.
+func (c *ChannelGraph) NewPathFindTx() (kvdb.RTx, error) {
+	if c.graphCache != nil {
+		return nil, nil
+	}
+
+	return c.db.BeginReadTx()
+}
+
 // ForEachChannel iterates through all the channel edges stored within the
 // graph and invokes the passed callback for each edge. The callback takes two
 // edges as since this is a directed graph, both the in/out edges are visited.
@@ -376,7 +386,7 @@ func (c *ChannelGraph) ForEachChannel(cb func(*ChannelEdgeInfo,
 // halted with the error propagated back up to the caller.
 //
 // Unknown policies are passed into the callback as nil values.
-func (c *ChannelGraph) ForEachNodeChannel(node route.Vertex,
+func (c *ChannelGraph) ForEachNodeChannel(tx kvdb.RTx, node route.Vertex,
 	cb func(channel *DirectedChannel) error) error {
 
 	if c.graphCache != nil {
@@ -414,7 +424,7 @@ func (c *ChannelGraph) ForEachNodeChannel(node route.Vertex,
 
 		return cb(directedChannel)
 	}
-	return nodeTraversal(nil, node[:], c.db, dbCallback)
+	return nodeTraversal(tx, node[:], c.db, dbCallback)
 }
 
 // FetchNodeFeatures returns the features of a given node. If no features are