Allow for Task to be `Ready`

[Ichmed]

Right now each task is kept as Pending even after completion so the executor can keep track of it, otherwise the task can not be restarted via alfad-ctl since the Executor will no longer poll the task.
If it can be assured that a task is no longer running (e.g. by tying a return value of Ready) to an update in the corresponding context, a new task could then be respawned when the "start" or "restart" action is performed over ctl.

This should not be implemeted if there is any way that a new task spawned via ctl corresponds to the same underlying task as one that is already running.

Note: it is not yet clear if removing the tasks from the executor would convey any performance benefit (might also depend on the executor implementation)