Add an operation to get the last-rendered image from whatever job received
such output from a Worker, and add a SocketIO subscription type to receive
updates about all jobs' last-rendered images.
These are necessary for a global last-rendered image display.
`SocketIOLastRenderedUpdate` objects are meant to be broadcast to SocketIO
clients (i.e. the web interface). They are sent to the job-specific room,
just like task updates.
There was no need to have the extra four letters 'nick', and some parts
of the code were already using just 'name' for the workers. This simplifies
and unifies things.
Remove the `shutdown` worker state. It was only used in Flamenco 2 to
signal to the worker that it should go offline. In other words, it didn't
use the target state in the request.
As of this change, just send `offline` as the requested state for the
worker if you want it to go offline.
Instead of always passing `status_requested` and `lazy_status_request`
in tandem, and requiring one when the other is present, they are now
always passed as a struct `WorkerStatusChangeRequest`. This struct has
both fields required.
`WorkerStatusChangeRequest` already existed, and is now altered by changing
its field `status_requested` to simply `status` -- the fact that it's
requested is already clear from the context.
This also introduces the 'lazy' status change boolean, which indicates
whether the status change should interrupt the worker's current task
(lazy=false), or only take effect after the task is finished (lazy=true).
When the web interface is showing the Workers list, it should no longer
get updates about jobs. A new subscription type `allJobs` is added, and
the `uuid` field of the subscription is no longer mandatory.
A task's "activity" field is a string that describes the last-performed
operation on the task. In Flamenco 2, a Worker's "last activity" is the
timestamp at which it was last seen. Using the same term for different
things is confusing, and thus gets removed until the Worker's "last seen"
functionality is reimplemented.
