Supervises the per-Repo writer queues that SQLite store mutations take their write slot from.

The :snodo_tasks_sqlite application starts this supervisor, so nothing is needed when the application runs. An application that lists :snodo_tasks_sqlite under :included_applications, or starts the VM without starting applications, must start it itself, once per node:

children = [
  Snodo.Extensions.Tasks.Store.SQLite.Supervisor,
  MyApp.Repo
]

Without it, every store mutation returns {:error, {:application_not_started, :snodo_tasks_sqlite}}.