DelegationManager.MakeWorkerSourceEvent for the main loop
handling an event may require an update to a sentence in the exec state, e.g. when a feedback is received
val worker_available :
jobs:(job_handle * Sel.Event.cancellation_handle * job_t) Queue.t ->
fork_action:(job_t -> send_back:(job_update_request -> unit) -> unit) ->
feedback_cleanup:(unit -> unit) ->
delegation Sel.Event.t