Remove broken use of clock.call_later
Remove broken use of clock.call_later

background_updates was using `call_later` in a way that leaked the logcontext
into the reactor.

We could have rewritten it to do it properly, but given that we weren't using
the fancier facilities provided by `call_later`, we might as well just use
`async.sleep`, which does the logcontext stuff properly.

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