This is half of the work: this will load the threaded receipt for each
thread, every time we add/update a timeline item for an event that had a
thread summary. Since we don't know which of the private or the public
receipt is the most advanced, we simply pass both, to start with; it's
expected that this code dies later, when we fold it in into the event
cache.
The second half will consist in updating the thread summaries when a new
read receipt event happens.
Benjamin Bouvier
8ae4acdc00