New sentinels are only created when the RoomMember state changes, so we don't
needlessly deep copy RoomMembers f.e. MatrixEvent. Sentinels co-exist with
RoomState.members which are single instances to which listeners can be attached.
This gets the best of both worlds (don't have to keep re-attaching listeners on
member changes, don't have needless memory consumption).
Accessed via MatrixEvent.sender property. Deep copy the list of state events
from initial sync for old/current RoomState so updating the .sender property
doesn't affect both of them. Reverse the insertion of initial sync MatrixEvents
so state diverges to *earlier* points in time. Add a 'forwardLooking' property
to MatrixEvent to determine which out of 'content' and 'prev_content' entities
should take into account e.g. when determining display names. Finally, always
create new RoomMembers when they are updated in order to prevent corrupting
(read: sharing the same member object) the MatrixEvent.sender property of
existing events in the timeline.
This makes it easier to localise in the future (if name == '?'), whilst
still indicating an unknown name. Ideally we would be returning null, but
this would require null checks every time the room name was accessed, which
isn't ideal. This also makes the UT for this less brittle (rather than
comparing literal english strings which are prone to breakage when we
inevitably change 'Unknown' to 'unknown' or something else.
