Speed up course refresh further

[mpartel]

With over a hundred of exercises, it's still pretty slow.

[mpartel]

We could get some improvements by parallelizing

• reading @Points annotations
• making zips

Still, the most time is spent in Ruby code. Must profile.

[jamo]

I would say this was fixed by 29e10b5
i.e. lazy unlocks.

[mpartel]

That fixed a huge regression but refresh can't still be considered "fast". It's lowpriority though, if it can be made asynchronous.

[jamo]

Parallelizing creating zips would speed it up -- or add cache for not redoing zips when exercise has not changed would speed it up.

The refresh will be somewhat redone with langs integration so this is likely to be mitigated with it