Remove logcontext problems caused by awaiting raw deferLater(...)

[MadLittleMods]

Remove logcontext problems caused by raw deferLater. 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 #18905

It's unclear why we're only now seeing these failures happen with the changes from #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.

Dev notes

Our logcontext docs: docs/log_contexts.md

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
  • Feel free to credit yourself, by adding a sentence "Contributed by @github_username." or "Contributed by [Your Name]." to the end of the entry.
• Code style is correct (run the linters)

[MadLittleMods]

Since the goal is to wait forever, let's actually wait forever instead of 2**16 seconds.

[MadLittleMods]

We could have also fixed this by wrapping the deferred with make_deferred_yieldable(...). (see our logcontext docs

ex. await make_deferred_yieldable(deferLater(self.reactor, 1, lambda: None))

Instead, I've opted to replace all of our deferLater usage with more proper clock utilities that handle logcontext rules already.