Previously, events for which the sender, target (state_key) or remover
was missing from the room members would have their profile fetched
from network when registering the event into models.
This could cause very slow past events loading times for rooms, since
the event registering function (which contained the profile retrieval
directives) is run sequentially event-by-event.
Missing profiles are now lazy-loaded when events come into the
user's view in the QML timeline.
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