Make progress bars work properly

[glamperd]

One bar (CeremonyProgress) is for progress through the list of circuits. The other (StepProgress) tracks progress of the current step, while computing: download; compute; upload.

Compute progress may need to be estimated somehow.

[glamperd]

This will be a little more complicated. Downloads, and uploads are done in parallel with compute. We'll need a way to present this.