Remove the call to /keys/changes when we do an incremental syn
where the old sync token doesn't match the one in the device list
store. To allow us to do this, always save the device list store
before saving the sync data, so we can safely assume the device
list store is at least as fresh as the sync token in the sync store.

Thread save functions through to allow this, add a delay parameter
so the sync can save the device list immediately and skip the wait,
and add a wantsSave() method so the sync can skip saving the device
list if the sync store isn't going to save anyway.

Fixes element-hq/element-web#6068

If save is called with a delay that would want the save to happen
sooner then the save we currently have scheduled, cancel the
current save and schedule a new one for the sooner time.