... and a convenience method in PerSessionKeyBackupDownloader to access it.

... so that we don't need to remember to do it everywhere.

This doesn't have any effect on the tests, but *does* stop jest from hanging
when you run the tests in in-band mode. It shouldn't *really* be needed, but
using fake timers gives more reproducible tests, and I don't have the
time/patience to debug why it is needed.