Rapidly triggering new artworks is causing multiple parallel downloads

[jvolker]

As described in #64 if downloads are triggered rapidly causes in multiple simultaneous downloads and leads to chaos. The same issue can occur if the frame looses and reestablishes the connection to the pubsub server: https://github.com/OpenframeProject/Openframe/issues/64#issuecomment-539232667

There a partial fix available in https://github.com/OpenframeProject/Openframe/pull/65#issuecomment-541322039:

0cce2a5 fixes some of #64 (comment). It covers situations where the frame has started a download and cancels if a new download is requested. But it doesn't cover situations where the new download has been requested so quickly that the first download hasn't even started yet.

So it's not entirely fixed yet and needs some more work.