This patches add handling for unseen messages counter, and last message received. The
counters are retrieved from DB on startup, they updated in-memory. Frontend are expected
to have their own in-memory counter too, the backend one is used for re-sync (e.g.; on page
reload).
Bridge methods are added to retrieve the state, and indicate which message has been seen
last by user. A signal is sent on counter reset to all frontends can synchronise.
This is currently local only, but it should be compatible with a future XEP-0490 (Message
Displayed Synchronization) implementation.
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