web/dashboard: optimize task list auto-refresh

[buchanae]

The auto refresh on the task list is a excessive. Looks like it refreshes every 2 seconds, and in my dashboard that's download over 500KB per request.

I do like the idea of having the dashboard update without needing to hit refresh, but there's a lot of room for optimization here:

• increase the interval to at least 10 seconds
• use the page visibility API to only update when the tab is active (i.e. don't bother refreshing when the user isn't looking)
• refresh tasks individually, and cache tasks in a final state
• expose a task events API via HTTP and subscribe to that, pushing polling/streaming down a layer in the stack
• after the initial load, only state changes, so refreshes can use the minimal task view

https://github.com/ohsu-comp-bio/funnel/blob/master/webdash/app.js#L51

[buchanae]

The main ideas left here are:

• allow GetTask and ListTasks to request a specific subset of fields in the response to enable the client to get exactly what it wants without extra weight
• provide an event system which client can easily subscribe to
• server side rendering of initial page load
• configurable and pause-able refresh in the dashboard

I'm not sure any of those are worth the amount of work and complexity. The initial problem was improved a lot.

The configurable and pause-able refresh loop is a good idea, but let's address that in a separate issue later.