Rewrite the server as stateless; define REST APIs

Remove redis and websocket-based architecture
Introduce simple express server with REST APIs
APIs included now:
/version - to get the version information of the server
/page/:language/:title - Fetch the page with given title
and language. The output contains segment annotated page content

Allow CORS for now.

Change-Id: I120cb20fe672565343d8223312947ee55e211a9c