remove unused commented out code
server-sent events over websockets preparations

- add dws_websocket_manager: websocket-session registry
- improve dws_websocket_handler to init/update/drop
  websocket-session registry in all the necessary cases
- improve distributed database initialization to support
  multiple tables (not just session)
- session table renamed to dws_session
- preparations for lock manager (dws_lock table)
add simple dws_lock_manager
service broker state transfer eunit timeout
a proper message encoding for our SSE-over-WS