Worker plugin is not notified when worker releases task

[nre]

The worker plugin is notified when a task changes state but not when the task is released.

@mrocklin asked whether release_key should be added to the worker plugin API.

Alternatively, release_key could notify the worker plugin that the task has transitioned to "forgotten", which is consistent with the behaviour of the scheduler plugin.

I am keen to implement either solution.

[jacobtomlinson]

I think that given the worker is still working on the task, despite it being released, means that moving to forgotten doesn't make sense.

A PR to add release_key to the worker plugin would be a nice addition.