This is some preparatory work for fixing up the problems with te timeline
ordering of unsent messages
(https://github.com/vector-im/vector-web/issues/1120). The functional changes
here should be minimal (bar an extra `Room.localEchoUpdated` when the local
echo is first added to the timeline).
Give `MatrixClient.sendEvent` its own entry point `Room.addPendingMessage`
instead of pushing it through `addEventsToTimeline`; this considerably
simplifies the implementation of the latter and also means that we can contain
the `_txnId` ming to MatrixClient.
Move the code which deals with a successful `/send` response from
`MatrixClient` into `Room.updatePendingEvent`, since it involves fiddling with
the innards of the Room.
Also adds a new EventStatus 'SENT' for events which have been successfully sent
but whose remote echo we still haven't received.
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.
