Reload the timeline when we get a gappy sync
After a resume from sleep, we don't want to start paginating through the whole
timeline. If we were in 'stuckAtBottom' mode before, treat a gappy sync
similarly to how we handle the 'scroll to bottom' knob, which is to reload the
timelinewindow starting from scratch.
Fixes https://github.com/vector-im/vector-web/issues/932, and will probably
also fix https://github.com/vector-im/vector-web/issues/943, though I haven't
tested it yet.
