This test doesn't compile in a way that reveals a problem with the design. The
verification service takes a `Request<'msg>` parameterized by the message
lifetime, and returns a future unconstrained by the message lifetime (it hashes
upfront to avoid requiring that `'msg` outlive `call`). But the `Batch`
middleware has the verification service working on its own task, so how can we
ensure that the message lives long enough to be read by the worker task?