MessagesModel: Implement check for messages duplication

[Kaffeine]

"Clients wishing to determine whether a new message with the scrollback header matches a previously-logged message with the same message-token SHOULD compare the message's sender, contents, message-sent or message-received timestamp, etc. Note that, in XMPP, the server only supplies a timestamp for scrollback messages, not for messages received while you are in a room; thus, non-scrollback messages will lack a message-sent timestamp."