Because of instantiating new ruby script engine take a lot of time, zengine uses single ruby script engine instance and take care of thread safety with synchronized block. [ZEPPELIN-86](https://zeppelin-project.atlassian.net/browse/ZEPPELIN-86)

Currently, zengine does not expect there're ruby script engine instance more than one. Also except ERBEvaluator, no other class need to access rubyScriptEngine.