Workers can now be soft-deleted. Tasks assigned to the worker will remain
associated with that Worker. Active tasks will be re-queued so other
workers can pick them up.
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.
rFcfb17b178da2055ef12b2aa2ad8f7f778a952bc3 changed the semantics of
`SocketIOWorkerUpdate`, in the sense that any update that doesn't change
the worker status can omit `previous_status`. This commit adjusts the
unit test for this.
