This provides optional support for fetching old events via the /context API,
and paginating backwards and forwards from them, eventually merging into the
live timeline.
To support it, events are now stored in an EventTimeline, rather than directly
in an array in the Room; the old names are maintained as references for
compatibility.
The feature has to be enabled explicitly, otherwise it would be impossible for
existing clients to back-paginate to the old events after a gappy /sync.
Still TODO here:
* An object which provides a window into the timelines to make them possible to
use. This will be a separate PR.
* Rewrite the 'EventContext' used by the searchRoomEvents API in terms of an
EventTimeline - it is essentially a subset.
We want to emit AFTER starting the timers so tests can speed
up time. We also want to do this because clients may want to
retryImmediately() on sync errors (which would be lost unless
the timer had already been started)
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.
Add polyfills for Array.map/filter according to MDN because it looks much
better than the utils format.
Add stub tests for edge cases and implement test for the common case.
