Resource monitoring messages

I'm told there is already some work in progress on adding a message type for resource monitoring ('resource_request', 'resource_response') to check on available memory, CPU usage, etc. Following up on an in-person conversation with @ivanov, some thoughts:

* Should this same communication channel provide some static information like `multiprocessing.cpu_count()`? We might draw inspiration from [the informative `_repr_html_` of `dask.distributed.Client`](https://github.com/dask/distributed/blob/master/distributed/client.py#L582).
* It would be cool to support a notebook / JupyterLab extension to display (and actively update) visual indicators of memory and CPU usage on the machine where the kernel is running.

I'd be happy to write some code for this, but I don't want to step on any existing branches. If these comments would fit into an existing GH issue, feel free to close this one and direct me there.