Allow (un)block_room storage functions to be called on workers (#18119)

This is so workers can call these functions.

This was preventing the [Delete Room Admin
API](https://element-hq.github.io/synapse/latest/admin_api/rooms.html#version-2-new-version)
from succeeding when `block: true` was specified. This was because we
had `run_background_tasks_on` configured to run on a separate worker.

As workers weren't able to call the `block_room` storage function before
this PR, the (delete room) task failed when taken off the queue by the
worker.