graph/db: add version parameter to IsPublicNode

Make IsPublicNode version-aware by routing to the appropriate SQL
query based on gossip version. V1 and v2 have different criteria for
determining node publicity (v1 requires four signatures, v2 requires
one). Convert TestNodeIsPublic to versioned test for both protocols.

graph/db/kv_store.go

@@ -4005,7 +4005,13 @@ func (c *KVStore) FetchChannelEdgesByID(chanID uint64) (
 // IsPublicNode is a helper method that determines whether the node with the
 // given public key is seen as a public node in the graph from the graph's
 // source node's point of view.
-func (c *KVStore) IsPublicNode(pubKey [33]byte) (bool, error) {
+func (c *KVStore) IsPublicNode(v lnwire.GossipVersion, pubKey [33]byte) (bool,
+	error) {
+
+	if v != lnwire.GossipVersion1 {
+		return false, ErrVersionNotSupportedForKVDB
+	}
+
 	var nodeIsPublic bool
 	err := kvdb.View(c.db, func(tx kvdb.RTx) error {
 		nodes := tx.ReadBucket(nodeBucket)