Show onging tasks in worker rows on status and tasks tabs of the bokeh dashboard

I find it confusing that the tasks are a displayed on the status/tasks tabs as horizontal color rectangle only once they are complete. For instance, it can give the misleading impression that a worker is not doing anything while it is actually processing a task that lasts 10x the average time of tasks completed concurrently on other workers.

I think it would make things clearer to display the task rectangles as soon as the execution of the task starts on the worker and thereafter extend the righthand side of ongoing task rectangles whenever the dashboard is refreshed (e.g. every 200ms second or so). I am not sure if this can be done efficiently with bokeh or if it would require sending additional events from the workers to the scheduler. Ideally, the right-hand side border of rectangles for the ongoing tasks could be displayed differently (e.g. a transparent right-hand side border)

This is somewhat related to #1883 but different. I cross-link so that the person (maybe myself) tackling this keep the two issues in mind when fixing any of them.