This is a normal
problem where we `await` a deferred without wrapping it in
`make_deferred_yieldable(...)`. But I've opted to replace the usage of
`deferLater` with something more standard for the Synapse codebase.
Part of https://github.com/element-hq/synapse/issues/18905
It's unclear why we're only now seeing these failures happen with the
changes from https://github.com/element-hq/synapse/pull/19057
Example failures seen in
https://github.com/element-hq/synapse/actions/runs/18477454390/job/52645183606?pr=19057
```
builtins.AssertionError: Expected `looping_call` callback from the reactor to start with the sentinel logcontext but saw task-_resumable_task-0-IBzAmHUoepQfLnEA. In other words, another task shouldn't have leaked their logcontext to us.
```
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