Change calculating display names from O(n^2) to O(n)
Reduces initial sync times from ~30s to ~1s on accounts with heavily
populated rooms.
The problem was that f.e. RoomMember it would try to calculate the
display name, which involved looping each RoomMember to get their
display name to check for disambiguation. We now cache display names
to user IDs so we don't need to loop every member when disambiguating.
