graph/db: remove GraphCacheNode interface

With the previous commit, the AddNode method was removed and since that
was the only method making use of the ForEachChannel on the
GraphCacheNode interface, we can remove that method. Since the only two
methods left just expose the node's pub key and features, it really is
not required anymore and so the entire thing can be removed along with
the implementation of it.

graph/db/graph_cache.go

@@ -6,33 +6,10 @@ import (
 
 	"github.com/btcsuite/btcd/btcutil"
 	"github.com/lightningnetwork/lnd/graph/db/models"
-	"github.com/lightningnetwork/lnd/kvdb"
 	"github.com/lightningnetwork/lnd/lnwire"
 	"github.com/lightningnetwork/lnd/routing/route"
 )
 
-// GraphCacheNode is an interface for all the information the cache needs to know
-// about a lightning node.
-type GraphCacheNode interface {
-	// PubKey is the node's public identity key.
-	PubKey() route.Vertex
-
-	// Features returns the node's p2p features.
-	Features() *lnwire.FeatureVector
-
-	// ForEachChannel iterates through all channels of a given node,
-	// executing the passed callback with an edge info structure and the
-	// policies of each end of the channel. The first edge policy is the
-	// outgoing edge *to* the connecting node, while the second is the
-	// incoming edge *from* the connecting node. If the callback returns an
-	// error, then the iteration is halted with the error propagated back up
-	// to the caller.
-	ForEachChannel(kvdb.RTx,
-		func(kvdb.RTx, *models.ChannelEdgeInfo,
-			*models.ChannelEdgePolicy,
-			*models.ChannelEdgePolicy) error) error
-}
-
 // DirectedChannel is a type that stores the channel information as seen from
 // one side of the channel.
 type DirectedChannel struct {
@@ -124,16 +101,13 @@ func (c *GraphCache) Stats() string {
 }
 
 // AddNodeFeatures adds a graph node and its features to the cache.
-func (c *GraphCache) AddNodeFeatures(node GraphCacheNode) {
-	nodePubKey := node.PubKey()
+func (c *GraphCache) AddNodeFeatures(node route.Vertex,
+	features *lnwire.FeatureVector) {
 
-	// Only hold the lock for a short time. The `ForEachChannel()` below is
-	// possibly slow as it has to go to the backend, so we can unlock
-	// between the calls. And the AddChannel() method will acquire its own
-	// lock anyway.
 	c.mtx.Lock()
-	c.nodeFeatures[nodePubKey] = node.Features()
-	c.mtx.Unlock()
+	defer c.mtx.Unlock()
+
+	c.nodeFeatures[node] = features
 }
 
 // AddChannel adds a non-directed channel, meaning that the order of policy 1