We need to reset things at the UI level when we get a gappy sync, so give the
clients something to listen for.
Also add a bunch of tests for that bit of code.
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.
It was possible for the test to end (via done()) before the final /events req
was flushed, resulting in a fail as there were requests outstanding. We now wait
until the final flush is done before done()ing.
