Avoid creating a separate base db instance for graph database (#221)

* Avoid creating a separate base db instance for graph database

* Fix failing test file build

* Use getter method to get base database

packages/util/src/database.ts

@@ -86,6 +86,10 @@ export class Database {
     });
   }
 
+  get conn (): Connection {
+    return this._conn;
+  }
+
   async init (): Promise<Connection> {
     assert(!this._conn);