new:
1.add user online state cache in global, and check state before
  each time RPC try to receive msg, if user is offline, then cancel
  the reception and return, in case the msg is wrongly received.

modification:
1.change index.js to add bidirectional para in delete friend request,
  find a bug in friends views.py which will cause incorrect delete the
  other half of the friend relationship in database, to be fixed.