Change Message into unmutable class
Currently the only thing that ever changes in Message is its delivery state. It is only interesting for one part of code - that displays messages, and only when messages are fresh and not loaded from history. It could/should be moved into new service that manages which delivery states are of our interest (mapped per-message-id of some kind) and then Message class could be greatly simplified. Junior job: 0
(from redmine: created on 2014-02-07)