Linearizer can get stuck due to cancellation

[squahtx]

Once a Linearizer task acquires the lock, it does a 0-duration sleep to reset the call stack:

synapse/synapse/util/async_helpers.py

Line 466 in 605d161

return self._clock.sleep(0)

In the rare event that the task is cancelled while sleeping, the lock is not released and the Linearizer becomes stuck.

Can be reproduced by modifying the cancellation test slightly. Cancelling the 2nd task after 1st task releases the lock (instead of before) will make the test get stuck indefinitely:

        with cm1:
            pass

        d2.cancel()

instead of
https://github.com/matrix-org/synapse/blob/release-v1.53/tests/util/test_linearizer.py#L159-L162

[squahtx]

related to #3528

[squahtx]

A good place to start would be to convert Linearizer to async code, reversing 638d35e.
This is okay since we no longer support twisted < 18.7.