The Task details component already linked to the Worker it was assigned
to last, and now the Worker links back to the task.
There's only one task shown in the Worker details. If the Worker is
actively working on a task, that one's shown. Otherwise it's the
last-updated task that was assigned to the worker.
This fixes a bug where 'Worker undefined changed status' was logged in
the web interface, as that was (back then incorrectly) `workerupdate.name`.
Now that code is correct.
A worker state change request can now be cancelled by requesting the worker
to go to its current state. In other words, a previously requested change
`A → B` can be cancelled by requesting the worker goes to state `A`.
Previously this would simply overwrite the last request, resulting in a
requested state change `A → A`. Having this non-lazy would even interrupt
the currently running task.
