D3: Async Arc Progress Gauge / Meter
The progress meter is based on the number of files that have downloaded. In our case,
numberOfFiles is set to 20.
We then generate 20 random durations between 0 - 3000ms to simulate download times, and call
setTimeout using the durations. Each timer then calls
update, which increments the number of files downloaded, increasing the progress arc with a tweening function.