Returns a specification to start this module under a supervisor.

See Supervisor.

@spec run_async(GenServer.server(), (-> term())) :: :ok

Queues fun behind the work already queued, without waiting for it.

@spec run_on(GenServer.server(), (-> result), timeout()) :: result when result: term()

Runs fun on the worker queue once the work already queued is done. Called from the queue itself, it runs right away. Waiting longer than timeout exits the caller; fun stays queued and still runs.