Add an option to limit incoming unread messages on update

[Kaffeine]

It is possible to have thousands of unread messages and it makes little sense to actually spam them out during one of the connection step (updates receiving).

Right now it can hang Telepathy Client for a while and we don't want such behavior at all — the proper solution is to expose Dialogs telepathy interface and say (via DBus) that we have N new messages in M dialogs and maybe send the content of the last message.

We need Dialogs API and Service-side Logging API to handle "thousands of unread messages on connect" situation properly.