FIX-898: initial commit and test fix

[AnthonyKot]

The initial attempt to display correctly number of unread messages and handle deleted message edge case.

[or-else]

Sorry, but I don't think it's an acceptable fix - it's too expensive. You are going to read potentially multiple large messages from the DB for each read receipt. That alone would likely generate DB load comparable to everything else combined, particularly if there is a lot of large group topics.

[AnthonyKot]

I agree, let's treat this as a draft solution. My plan is to verify it locally and then work on adding a new query to MongoDB to efficiently retrieve the count of unread messages.