Pass run_in_background a callable; don't call it

[anoadragon453]

Follow-up to #134

It helps if we use the function correctly:

Traceback (most recent call last):
  File "/home/synapse/src/synapse/logging/context.py", line 871, in run_in_background
    res = f(*args, **kwargs)
          ^^^^^^^^^^^^^^^^^^
TypeError: 'coroutine' object is not callable