graph/db: detach forEachNode from KVStore

So that we can use it in a context that only has access to a
kvdb.Backend. We'll use this in our kvdb->SQL migration.

Author: ellemouton

graph/db/kv_store.go

@@ -658,7 +658,7 @@ func (c *KVStore) ForEachNodeCached(cb func(node route.Vertex,
 	// We'll iterate over each node, then the set of channels for each
 	// node, and construct a similar callback functiopn signature as the
 	// main funcotin expects.
-	return c.forEachNode(func(tx kvdb.RTx,
+	return forEachNode(c.db, func(tx kvdb.RTx,
 		node *models.LightningNode) error {
 
 		channels := make(map[uint64]*DirectedChannel)
@@ -774,7 +774,7 @@ func (c *KVStore) DisabledChannelIDs() ([]uint64, error) {
 // executed under the same read transaction and so, methods on the NodeTx object
 // _MUST_ only be called from within the call-back.
 func (c *KVStore) ForEachNode(cb func(tx NodeRTx) error) error {
-	return c.forEachNode(func(tx kvdb.RTx,
+	return forEachNode(c.db, func(tx kvdb.RTx,
 		node *models.LightningNode) error {
 
 		return cb(newChanGraphNodeTx(tx, c, node))
@@ -788,7 +788,7 @@ func (c *KVStore) ForEachNode(cb func(tx NodeRTx) error) error {
 //
 // TODO(roasbeef): add iterator interface to allow for memory efficient graph
 // traversal when graph gets mega.
-func (c *KVStore) forEachNode(
+func forEachNode(db kvdb.Backend,
 	cb func(kvdb.RTx, *models.LightningNode) error) error {
 
 	traversal := func(tx kvdb.RTx) error {
@@ -819,7 +819,7 @@ func (c *KVStore) forEachNode(
 		})
 	}
 
-	return kvdb.View(c.db, traversal, func() {})
+	return kvdb.View(db, traversal, func() {})
 }
 
 // ForEachNodeCacheable iterates through all the stored vertices/nodes in the