Add missing await to sleep calls

[anoadragon453]

Reported by @realtyem in DMs. The consequence was that logcontexts became lost for these code paths:

synapse.http.server - 129 - INFO - sentinel - <SynapseRequest at 0x747656210a50 method='POST' uri='/_matrix/client/r0/account/password/email/requestToken' clientproto='1.1' site='test'> SynapseError: 429 - Too Many Requests (rc_3pid_validation)
synapse.logging.context - 1003 - WARNING - sentinel - Calling defer_to_threadpool from sentinel context: metrics will be lost
synapse.logging.context - 91 - WARNING - sentinel - Expected logging context POST-458 was lost

Missed in #18546 and #18595.

cc @erikjohnston

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[erikjohnston]

"oops" 😬

[anoadragon453]

CI failures are a flaky test. Merging manually.

[MadLittleMods]

Is this something we can lint?

We already detect this kind of thing for async/await

error: Value of type "Coroutine[Any, Any, None]" must be used  [unused-coroutine]
note: Are you missing an await?

[erikjohnston]

Oh mmm, we probably want to change the return type of sleep

[erikjohnston]

I did this here: #18772