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.
