sqldb: define schemas for all graph node tables

In this commit, the various SQL schemas required to store graph node
related data is defined. Specifically, the following tables are defined:

- nodes
- node_extra_types
- node_features
- node_addresses

sqldb/migrations_dev.go

@@ -2,4 +2,10 @@
 
 package sqldb
 
-var migrationAdditions = []MigrationConfig{}
+var migrationAdditions = []MigrationConfig{
+	{
+		Name:          "000007_graph",
+		Version:       8,
+		SchemaVersion: 7,
+	},
+}