Provide efficient way of publish an array of events

[rickmak]

In chat plugin, we need to publish many event on one message update. In current Hub.publish method. it will connect and close the ws connection on every call. We should provide an interface which is accept an array of events and send to skygear-serve in one ws connection.

refs: https://github.com/SkygearIO/chat/pull/114/files#diff-4f571f88d938ce25629fe3c296a561deR100